I don't know much about MSSM but BU is definetely not the place to go if this is your choice. When I was at BU seemed very cold and impersonal. Plus from what I hear they work their students much harder than at other schools. You may think that thats a good thing but in reality you won't learn more at BU, you will just be more stressed.
BU, i like the quieter city of boston and you can't go wrong with the BMC- you'll see a lot at that hospital. But if bigger, and more action (and open later) in a city is what you want, MSSM might be better for you.