What is the difference in the two in terms of party/social scene? Which is known more as a party school?
MSU has more of a party reputation. MSU is famous for couch burning, but Michigan has had its share of wild incidents like the infamous frat boy trashing of a Treetops lodge in Gaylord, and some very excessive frat initiations. Michigan has plenty of parties, but they are not ubiquitous, and you can avoid them if you wish (especially on north campus). On football Saturdays the MSU campus around the stadium is always trashed, not so at Michigan.
There are plenty of parties at both, but the Michigan State girls look better and the parties are usually more fun, in my experience. I am sure you can have as much fun at Michigan, but you may have to look a bit harder.