Vtyjkyuk

If the part of mountain bikes that you think looks cool is the rear suspension, I very much recommend you buy an uncool bike. Rear suspension is very heavy, requires maintenance, means you can't fit good mudguards, stops you using panniers and means that, while you're riding, part of your energy is going into flexing the bike around instead of moving you forward. It's also completely unnecessary unless you're riding down mountain tracks at high speed, which you say you won't be.

I would suggest a "hybrid" (also known as a "fitness bike" or "commuter bike") – they have a more-or-less mountain-bike-shaped frame and might have front suspension (which is a comfort-versus-weight trade-off that you'll have to consider). They have narrower, smoother tyres than mountain bikes, and these are better suited to riding on paved surfaces but they'll still be OK for light off-road use such as gravel tracks.

Generally mountain bikes offer certain advantages and disadvantages over, say, road bikes for commuting.

Advantages:
The seating position is comfortable, and ridig will also be very comfortable due to suspension and the wider tires.
Also, in bad weather, mud and snow you probably have better traction with MB tires. Also the wide handlebars make nimble riding very easy.

Disadvantages:
The wider tires, higher weight and less aero positioning of the rider mean that you generally have to invest more power to go the same speed.

If a bike, that looks 'cool' to you means that you'll ride it more, then go for it. But it might not be the best possible buy. There's also bikes specifically made for commuting which focus on being very reliable and still comfortable (Canyon Urban and Commuter series come to mind).

To answer your question completely, we'd need to know how long your commute is, what your budget is and if you consider also riding trails with your bike.

I'm riding a hardtail 26" XC bike for commuting, never had a road bike, but tried once, so that's my experience:

Pros:

• more upright position which is good in traffic - you see more, you're more visible for cars


• a bit more comfort (highly depends on tires)


• possibility to ride anywhere (this too depends on tires)


• lower gearing - this is good if you live in a hilly place

Cons:

• bad fenders. Installing full fenders on mtb (especially with ammo-fork) is a pain


• more rolling resistance: even if you install slick tires, they are going to be wide (mine are 2.1")


• more wind resistance due to upright position - you'll have to push harder to ride as fast as people on road bikes


• lower gearing - the top speed for XC bike is lower than for road bike

One more to "pros", but very specific to where I live (Saint-Petersbourg, Russia) - there are lots of XC races, much more than road ones. So if you'd like to race one day - mtb may be better

There's more than just a mountain bike and a road bike

Especially if you consider commuting I would add at least one more option than the two you mention.

Road bike

Road bike are intended for a fast rides on a rather flat surface and I would say they aim more for a race riding so that's probably not what you want to have for a daily commuting. The position is not comfortable, it is optimised for limiting your drag coefficient and getting most power out of your muscles.
Such bike has a seat positioned higher than a handle bar, often with ram handles to enable even lower position (sometimes handle suspension in the middle). They have thin tyres and in general are very light. They often aren't fitted with fenders or even lights to save on weight.

Mountain bike (MTB)

These bikes are optimised for a difficult mountain paths where there are heavily changing conditions, steep slopes, mud etc. Yet the biker does not really care for that (i.e. getting dirt). They'll have a bit more comfortable position than a road bike, with seat just below the handlebar.
They will have wide tyres with deep tread, plenty of gears, often additional handles sticking up perpendicular to the main handle to enable shifting position when needed. Usually fitted with some sort of shock absorbers.

City bike

If you've ever been in Netherlands, you've seen plenty of those for sure. These bikes are designed to work in a typical city conditions. Dutch are freaks about bikes (I've been told an average Dutch has at least 4) and they really know what they are doing.
Typically city bike has a very upright position. It is far more comfortable than anything you can get in either road bike or MTB. Tyres are quite wide but with tread not so deep as in MTB. Handlebar is much higher than the seat. They often have a single gearbox in an enclosed case to reduce maintenance with 3, sometimes 5-8 gears. Also usually fitted with luggage space (sometimes also in the front), fenders, lights, protection so that your dress don't get into the wheel, chain cover and everything to ensure that a gentleman in his suit or a lady her dress don't get dirty and can get into the office or whatever other business they have.
These bikes are really heavy (20 kg is a typical weight) and intended for biking on a flat surfaces but handles small terrain obstacles too. Sometimes are equipped with shock absorbers, usually have very comfortable saddles. This is all about your comfort, not speed.

If you don't plan too much off-road I would really consider this option.

Cool bikes are everywhere

All types listed above have some cool bikes. You may need to look for it a bit more. I own a Dutch-style city bike that I really believe is cool (Batavus Mambo) but I had some MTBs and road bikes in the past as well.

What I would consider is making sure your bike is comfortable for you. Otherwise you'll just stop using it way too soon.

in addition to the other replies I'd like to emphasize:

• the mountainbike makes potholes less of an issue. Partly due to the actual suspension, but also from the suspension provided through the tires, and by the larger inertia of the tires. So while no bike will instantly disassemble under you when going through a pothole, you probably need to be react to them on road bikes. While on a mountainbike you're way further away from losing control even if a pothole hits you completely unexpected.




• tram tracks: If you ride in a town with trams, wider tires have the advantage that they don't slide into tram tracks as easily as narrow tires. Even if your mountainbike tires are not wider than the tram track, you usually need to hit them at a very steep angle to get into trouble. Road bikes are less forgiving.

So these points are pro mountainbike: You need to worry less about the ground ahead of you and can spend more attention on traffic. BUT this should only be a comfort argument. If the road and traffic are so bad that they are a real issue on a road bike, then maybe riding there isn't such a good idea on a mountainbike either...

Con:

• In city traffic, I experience that the most tiring aspect is the constant stop and go. I.e. all the energy you put into accelerating to cruising velocity is gone by the next stop sign / traffic light (or for riders who prefer to adapt their driving to actual traffic rather than road signs: "is gone the next time you give priority to another participant of road traffic"). Accelerating rotating mass takes more energy than non-rotating mass, so I claim this takes an extra toll on how tiring the ride is that may be more relevant than air drag (although looking through other threads on bicycles.stackexchange, this seems to be matter of dispute).

For similar riding, I actually prefer the mountain bike frame over a road bike. I like the stability of the wider tires, and ability to cut across grass or gravel areas without a second thought. However, I look for a few specific characteristics:

• The bike should only have front suspension (no rear, too much weight for not enough benefit), and maybe a seat spring.




• Look for tires with a smoother tread pattern. This will make for a much smoother and easier ride, while still providing extra traction and stability for places where the road might not be so nice.




• More gears aren't necessarily better. The drive on a mountain bike is likely to have some extra low gear ratios for easier work in tough terrain that just aren't as necessary for rides that are mainly on the road. Riding uphill on asphalt is just not the same punishment as riding up an actual mountain, and a bike with fewer gears is likely to result in easier, cleaner shifts. (Of course, there's also a "you get what you pay for" factor here).

And again, these are my preferences.

You can ride a mountain bike on the road (you might not be as efficient or able to keep up with people on road bikes though), but you really can't ride a road bike on a trail.

You asked if it is tiring, no, not really, so long as the seat is properly adjusted and you use the gears properly it will be fine.

There are things in between though, for example my first higher quality bike (that came from a bike shop as opposed to somewhere like Walmart) was a hybrid. Its tires were still fairly narrow but had ridges, but not as narrow as a road bike's. It had maybe an inch and a half of front suspension and no rear suspension. I was able to ride it on trails, but a real mountain bike with rear suspension was much more enjoyable once I got one. That hybrid was great though, I used it to commute from my college dorms to campus (in a busy city with potholes) so the suspension made it a lot smoother.

Lots of answers already, but i'll try to give my point of view.

I'm a Dutchie and regularly use both bikes, but each for different purposes. They each have their pro's and cons. MTB is a mid-end hardtail and city bike has a simple three-speed geared hub with an enclosed chain guard.

City bike

Pros

• Low maintenance


• Built-in lock


• Luggage rack/panniers


• Fenders


• More comfort during short trips (seating position/saddle)

Cons

• Takes more effort to go fast


• A bit boring

MTB

Pros

• Fun to ride


• Faster than a city bike


• More traction

Cons

• No chain guard


• No fenders


• No lock

In short: a mountainbike is more fun to ride, but a city bike is easier to live with and way more practical for daily use.

Another aspect that hasn't been mentioned is your tires. Mountain bikes typeically have knobby tires and road bikes have smooth tires. I have a mountain bike, and when I wanted to start training for a week long ride, I had no real option but to ride on roads (because we don't have 60mile long trails where I live). Road bikes run with a lot higher pressure in their tires which limits the amount of friction you have with the road. If you increase the pressure in the mountain bike tires ( knobbys) to their max, your ride will have an extra vibration because of the tread on the hard road, and will take a bit more effort.

One way to eliminate this issue and still not have to invest in an additional bike ( and figure out where to store that second bike) is to buy some slick tires for the wheels of your mountain bike. These will typically hold more pressure than your average mountain bike tires so you will get a smoother ride with less effort.

If you do a lot of both types of riding you can invest in a second wheel set and have both types of tires ready to go at a moments notice.

What others have said about frame types is certainly true, but this is a way to have a little of both if you are limited to one bike!
Have fun riding!

Others have given pretty solid answers but I would like to add that there is another kind of bike made for everyday commute apart from city bikes.

I believe they're somewhat "exclusive" to France in retail (but you can build one yourself very easily), we call them "vélo tout-chemin" which means "all-road bike" in contrast to mountain bike which we call "vélo tout-terrain" (all-terrain bike).

The concept is the following : a simple mountain-bike frame with only front suspensions and tires with thread depth variations : the middle is flat like road-bike tires and the sides are thick and ridged like mountain-bike tires), but I don't know how hard or easy it is to find that outside of France.

So with such a bike you can put less effort in riding on road than with a mountain bike while keeping a good look and not fearing potholes or rain. You also keep the ability to do light off-roading (like forest tracks for example).

Customize :

If there are no such bikes in retail where you live, you can achieve the same result by buying a simple mountain bike and a second set of quick-change wheels with city-bike tires or simply equipping bicycle-touring tires if you want a single set of wheels.

Buy the bike that makes your forwards progress as lightweight as possible: a road bike, not a mountain bike.

I long time ago thought that buying a "hybrid" bike was a good idea. Best of both worlds, that was my idea.

Soon later, I switched to 28mm slick tires. Rolling resistance problem solved!

A bit later than that, I switched the handlebar to a drop bar, with shifting using bar end shifters and with braking using those drop bar brake levers made for V brakes (which I eternally hated). Air resistance problem solved!

What in the end I was annoyed about was the suspension fork. Couldn't change that, so I built myself a genuine road bike.

I have ridden with the hybrid to road bike conversion and with the true road bike on many terrains. 28mm slick tires work just fine on most acceptable quality forest paths. MTB may be better in the mud or in soft wet sand, but how often do you encounter those situations? (Besides, ever heard of cyclocross? They use what are essentially slightly modified road bikes on harsh terrains.)

You said your riding is "urban", on long routes using normal roads. That's where a road bike really shines. If your routes include forest paths, you may want to switch those stupid 23mm tires to 28mm ones. You may want to do that anyway, as after fixing a flat 28mm tires require less pressure than 23mm tires. Also, 28mm buys you slightly more ground clearance, which is good.

Oh, and road bikes are really cool. If your idea of cool includes riding on harsh terrains (although you didn't say you are planning to do that), you can always buy a cyclocross bike which is essentially a slightly modified road bike.