1. Environment. I'm not a city person, and I just couldn't imagine living in Philly after I visited. Ann Arbor is the type of college town I have grown to love.
2. Housing. I have a wife (who is a chef) to think about, and the only on-campus housing at Penn is a furnished hi-rise with a kitchenette. Michigan has an entire apartment complex dedicated to students with families.
3. Facilities. I still have to visit Michigan, but if their CD is remotely accurate, they have the better building for my tastes/style.
4. Job placement. By every empirical measure I could find, Michigan grads get better jobs than Penn grads.
5. Clerkship/Academia. Michigan is a powerhouse at producing clerks and academics. Penn is improving, but I want to maximize my opportunities, and that means sticking with the proven commodity.
6. Community. The largest plurality of Penn students (if not a true majority) want to work in NYC after graduation. I don't, and I felt a little out of place around so many who did. Michigan sends a good number of grads all over the country, so I fouldn't feel so alone when considering employment opportunities in various regions, and I wouldn't be at a disadvantage when flying everywhere for interviews, while others just drive for 2 hours.
7. Fit. Michigan has some very notable professors in fields of study that am interested in, and journals dedicated to those ares. I also just really connect with their promotional materials - the sense of humor and personality that shines through through the text jsut really attracts me. They are a school that doesn't take itself too seriously.