Have you heard back from Penn? I though Penn sends out RD decisions in late March. At any rate, is there a cost element to consider? Assuming there are no cost concerns, you should also visit Penn before deciding. Michigan does not offer IR as a major, only Political Science, but the Political Science department at Michigan is one of the best in the nation. Generally speaking, when choosing between peer programs, if cost is not a consideration, I recommend going for fit. Try to visualize your experience at both universities and go for the one you think suits you best.