Like you and Jerew, I'm also facing a similar decision. And I'm also only attending UChicago's revisit (maybe I'll see you there!). UChicago's been a top choice for me from the beginning. I like its emphasis on research, grad student community (I love the idea that it's contiguous with the rest of campus), location (I like urban), small class size, intellectual atmosphere, and overall friendliness and vibe I got from the faculty/administration and students.
Cornell's facilities are impressive, but I'm not completely sold on PBL. I also didn't have much interaction with the administration/faculty during my interview day, so I was left a bit cold. The location in NYC is great (a little far from the subway), but I'd prefer being near the rest of a university's campus. Cornell felt just like a building (albeit a great one) in a city, and I prefer a university feel.
Michigan had the university feel, but it all felt overly "rah-rah" to me. The large class size was a turn-off, and I felt I clicked better with students at other schools. Also not a fan of being in a college town for another four years.
Obviously, your priorities could be completely different from mine... but there's my $.02... Hope this helps...