Strongly in favor of the mountain bike, I'll bet 50 bucks Kalium is just assuming the road bike is better without ever having the option, or he lives in a really flat area and is a very fit person.
I've been biking for 8 years around the city, in a huge city.

1) First goal should be don't die. Thus you should actually use the sidewalk as often as possible (regardless of whether that is legal, or is the law worth all your future experiences?) Bikes are incredibly dangerous, but awesome fun.

2) Second goal should be don't strain your back and feel constant pain. Keep an upright posture with a mountain bike, you will (a) see more of the world around you (b) get more delicious wind in your face (c) require a lower activation energy to decide to bike, since you won't need any special biking shorts, gloves or things other than a helmet.

3) Third goal is exercise. It goes without saying that a heavier bike (which you will never, ever notice because no options will be there to be compared, so no hedonic onus) will exercise you more.

4) Fourth goal is look good: an upright posture will help you with that.