If there's only a "slight edge" with the rankings...I wouldn't worry about it too much. In the high rankings, a lot of times it's more nuanced than anything. If you think you have a better chance at getting a TA/RA position at Michigan, I'd follow the money. In the end, both schools are going to give you really great degrees. Also, look at the cost of living in both areas. Atlanta is a bigger city, so it might be more expensive than Ann Arbor. Also, I know from experience that you can live in Ypsilanti which is a 10-20 minute drive to U of M for significantly less money.