Congrats on your acceptances!
I think I'd consider the opportunity costs of each school. Even though you would maybe have to take out loans at Harvard, look carefully at their job placements. Are they getting stellar jobs or similar positions to UM graduates? Your advisors could tell you where their past graduate students are working. One of my professors told me I should conisder what kinds of jobs the graduates are getting and if they are getting jobs at the top schools/research institutes (which means they are probably making more money than you would at other jobs). I'm sure the graduates of Harvard and Michigan are able to get jobs, but what kind of job do you hope to get? If you are considering industry jobs, like think tanks, as well as academia, you might want to ask if graduates have been hired at places like Mathematica or RAND.
You might even want to consider where you would rather live. It's 4-5 years of your life. Also, would you want to buy a car if you went to Michigan?
Is re-applying to funding at Harvard just a formality? I'd ask if people ever lose funding or if re-applying is just for administrative purposes. It might just be a minor hassle. Do students receive more funding as they progress in the program? Your advisor can probably answer those questions.