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.
So, about that bet. Was that meant seriously, and if so what evidence would I need to supply that I either (a) have tried both a mountain bike and a non-mountain bike or (b) either (i) live somewhere not really flat or (ii) am very fit?
On ChrisHallquist's post extolling the virtues of money, the top comment is Eliezer pointing out the lack of concrete examples. Can anyone think of any? This is not just hypothetical: if I think your suggestion is good, I will try it (and report back on how it went)
I care about health, improving personal skills (particularly: programming, writing, people skills), gaining respect (particularly at work), and entertainment (these days: primarily books and computer games). If you think I should care about something else, feel free to suggest it.
I am early-twenties programmer living in San Francisco. In the interest of getting advice useful to more than one person, I'll omit further personal details.
Budget: $50/day
If your idea requires significant ongoing time commitment, that is a major negative.