I don't know both universities, but if you look at different rankings, I think that Cornell is better than University of Michigan (at Ann Harbor ?) in overall reputation.
This is an opinion only based on rankings (somewhat controversial).
To choose well, you need to find the program where you think you will fit the best, the place where you'd rather be for 1 or 2 years, and financial consideration (such as funding) can also influence your decision.
And you should also decide if you'd prefer a MS or a Meng...