The McGill Residences website is a good resource to find out how they differ logistically (ie. co-ed, not co-ed, layouts etc.). I lived at Douglas Hall this year and it was the best experience of my life. It's known to be the "study" rez although that really didn't seem to be the case as far as I am concerned. It's a beautiful, historic building with a lot of common space and a very tight knit community. Absolutely loved it. Of the other upper rezes, Molson is known to be the party rez but don't get too hung up on stereotypes - it all depends on the people who end up there each year. Every rez will have parties and every rez will have students who study more than they breathe. If you go in with a positive attitude, you'll have fun no matter where you end up.
Also, the hill isn't really bad at all. Keeps the freshman 15 off.
Again, it's hard to say in terms of everyone at a certain rez. I knew people at Doug who went out 4 days a week and some who didn't go out a single time all year. It's all based on who you are, who your friends are and how often you want to (and can afford to) go out. I think Molson is better known for having parties in the actual rez whereas that wasn't the case at Douglas. Everyone at Douglas was a scholarship winner and so it was known to be a studious rez based on that fact (don't get me wrong, people partied there a lot too! There won't be a single rez at University that doesn't party).