Thoughts? Impressions from students at either school?
Having interviewed and been accepted to both, I can give you my perspective.
I really liked both schools, and students at both seemed pretty happy. They both have P/F grading the first two years, and decent-sized hospitals on-site. I think it depends on what patient population you're interested in serving. BU has a large immigrant population in their hospital, many of which cannot pay for your service (which I thought was really cool). Additionally, due to the location in South Boston, they have a pretty busy ER. They seem to have many opportunities for service abroad as well, which interested me.
The U of M, on the other hand, allows for rotations into rural communities through their RPEP (or whatever) program for extended periods. This would be very interesting as well, but entirely different than the BU experience. That said, the U of M is not solely focused on rural populations, this is just an option you can take advantage of if you so choose.
If you're looking to do a competitive residency, both schools seems to provide equal opportunities for it to happen. The other differences that stand out in my mind are the tuition and cost of attendance, which is lower at the U of M, and the US News Rankings. The rankings shouldn't be too important, but the U of M is ranked higher in both research and primary care.
Honestly, both are great schools and will provide many opportunities so you really can't go wrong.
