I think lots of people have this debate, which highlights how close the two schools are.
I think most people would agree with two things: 1) Michigan has a stronger program right now; better placements, strong tradition of advising and caring about Ph.D. students, etc. and 2) Brown is likely to improve over the next few years. They are also both good "full service" programs in terms of having good coverage even outside of development in case your interests change.
That said, if you look at the history of U.S. News Rankings (obviously not perfect, but maybe the best option) or other rankings, you will see programs in the top 20 almost never move; and, if they do, it's pretty slow.
In other words, there certainly is probability that Brown will be a better program in 5 or 6 years, but it's likely less than 50%. I know people in this position who have taken both; in my limited sample, more have chosen Michigan, but that may change.
Overall, I would say that if you're really unsure I would recommend Michigan, but if you have a gut feeling in the other direction, I would take it.