Ok well I am definitely qualified to answer this one. I have actually been a student at both campuses, so I dont have a real bias towards one or the other.
U of M is a very research heavy campus. The facilities are top notch, the instructors are amazing and at the top of their field. It is a great school, but as an undergrad it was cut-throat competitive (people sabotaged others' projects so they wouldnt pass the course....). I dont know how much of that carries over into the med school. The campus is pretty, much smaller than MSU's main campus, but nice. There are two nice libraries on the main campus, plus a designated medical library. Ann Arbor itself is (in my opinion) kind of a snobbish town....there are fun places to go and fun things to do, but they are pretty full of themselves in general. There are obviously exceptions to this, but the people on that campus are the reason I transferred out and over to MSU.
MSU is much more of a laid back vibe than U of M. It is very focused on producing clinicians, rather than academic physicians, and this is expressed in their teaching style and curriculum more so than at U of M. A con is that there is a lot of expansion going on right now with both the osteo and allo schools at MSU, so yes the students are in some ways guinea pigs. The teachers are great, the facilities not as nice as those you would find at U of M. East Lansing itself is much more of a community in my experience. It is a large town, I think the same size as Ann Arbor, and also a stone's throw away from the capital, Lansing. The people seem to be friendlier, still competitive but not anything that I would consider to be wrong. There was much more of a vibe of helping each other out than what I experienced at U of M.
These are kind of generalities I can make about the schools and areas. If you have any more questions, please feel free to PM me!