Tough choice there. I haven't interviewed at Michigan yet but they are a really good school. I did go to UPenn and I really liked them. Their sim lab is GREAT and they have a ridiculously high specialization rate.
You may want to try creating a list of pros and cons for each. Like include atmosphere, student happiness, friendliness of faculty, type of program, distance from home, traveling opportunities, cost, etc etc and rank each one on a scale or something like that.