I was ready to accept my Michigan offer today, but then I come back and check this thread and I have doubts again. I really liked the lab rotation idea too at Penn, although there's more faculty at Michigan I'm interested in than Penn.
I don't really have any uncertainty about wanting to work in genetics, I'd be really, really surprised if I winded up doing something else. I'm a little concerned about Michigan's program taking significantly longer than Penn's. Most Penn students finish in 4-5 years from a Bachelor's. I knew that Michigan's program would take longer but I didn't know that genetics students at Michigan took 6+ years. I've also heard qualms before this thread that Michigan's genetics reputation is on the decline. The first few posts convinced me that wasn't true but now I'm unsure again.