There are plenty of dudes and ladies in both places, and from my experience at colleges, party and college are synonymous. so the one difference is skiing. in missoula you can drive 15-20 min. to a really horrible ski resort, snowbowl, and trust me i grew up skiing there, so you can go to lost trail (1.5 hr) or big mtn (2.5 hr). on the other hand in bozeman, there is bridger (30 min) and big sky (1.5 hr). i would definitely choose bozeman from that.
but like i already said, you have to look at what you're interested in! for instance if you want to do engineering, don't go to um because they don't offer it.
um -
http://www2.umt.edu/catalog/areastud.htm
msu -
http://www.montana.edu/wwwcat/programs/prog1.html