As a UIC third year student, here are my views:
Rush seems to have a happier student body... they are supported by better staff, better administration, and possibly better courses in the first two years.
UIC has the edge, however, in the third and fourth year clerkships. Though we have pulled out of Cook for many clerkships, we still have that option (which, I may add, isn't always very enticing!), but we have MANY more hospitals from which to choose. We've pretty much traded their Illinois Masonic for our Cook. However, note that hospital affiliations change frequently! UIC has always had more of a choice, however, of hospitals.
I chose UIC for that reason. I went realizing that the first two years weren't going to be great. And they weren't. But the reason I wanted to be a doctor wasn't to study all that nonsense I learned the first two years (well, somewhat...) but really for the clinical experience , for which I believe UIC has the edge.
Also, UIC is about $40,000 cheaper for the four years.
Both schools are good, and you won't go wrong either way. Try to talk to students at both schools to get a sense of the first two years; the latter years are more difficult to get a feeling about, because we're all over the place, but we're not at school to be asked!
Best of luck!