Look at their course catalog for robotics stuff, and see how many classes you'd be able to take. Look up course websites from previous years.
Also, see how many labs they have, and find out the number of total people involved - this shows how many opportunities you'll have.
Judging entirely by the websites (which may or may not be updated or representative), it appears UPenn would be your better option. They have 2x the faculty and research projects going on. Their lab specializes in robotics, whereas at UMich it appears to be more controls oriented with a subset of robotics. On the UMich site, they have a picture of only two robots - one of them was actually built at CMU. And UPenn is only a ~2+ hour drive from CMU, giving you visiting/collaborative opportunities.