bayessays, I'm finishing up my undergrad at UNC right now and I can say it's definitely a smaller department than UMich. From being fairly active in the department it's been my experience that everyone is really great and helpful although I think some of the first year PhD sequences are especially difficult (theoretical statistics). I think our probability group may be slightly better than UMich and UMich is definitely better at machine learning, I think we have two people doing ML type stuff right now. Other than that I don't know a whole lot about UMich so I can't tell you about how they compare on little things but if you have any UNC related questions you can PM me and I'll be glad to help out.