IMHO, duke because it has better home fellowships (especially retina) and fellowship match. both UMich and Duke are hard to match at because they draw heavily from their own med schools
I've talked to 3 people who interviewed at both last year and all 3 without hesitation said Duke. That being said, Michigan is incredible and would be a great place to train as well. Can't go wrong with either. But Duke seems to be the consensus.