Well, I've heard Wisconsin is really strong in Computer Architecture and Databases. I also heard that it's not so good for things like Robotics, HCI, and AI. Ranking-wise Wisconsin is ranked higher than Michigan, but I don't think there's enough of a difference in relative strengths between the two departments to base your decision solely on that. You might want to ask around and see what Michigan is strong in (isn't their AI lab pretty strong?) and try to match that with your interests. If you don't have a clear cut interest right away, you can try to gauge how well rounded each department is.