And build quality of the wheel. Many MTB wheels are over built to withstand off-road conditions so there is a better margin but a well built road wheel (32 or 36 spoke) should be very reliable. I'm 220 lbs and ride 32 spoke road wheels on some very poor city roads including 6 railroad crossings everyday. I run 23mm tires because thats what will fit and don't even bother to slow for the rough stuff. Once I retension the factory built wheels I have no problems. Most other people who have hand built wheels or have had thier wheels retensioned will have similar experience.
If you are an urban commando type rider than an MTB is probably a better bike for you but if you have decent miles to ride on you commute a good road style bike (cyclocross, tourer or light tourer) will be faster.
Craig