I would say sandflea answered that pretty well...
Just to clarify - there are two tracks: the Chicago track and the Urbana track which all first years start out on. The Chicago trackers stay in Chicago usually and most of the Urbana trackers have to go to either Rockford or Peoria for M2-4 -- Primarily only the MD/PhD students continue in Urbana. So the break down is usually - 175 Chi-town, 50 Peoria, 50 Rockford, 25 Urbana.
On that location preference card you get with acceptance it asks you to designate first and second choices for the two tracks - also it asks you to rank order 1,2,3 the other campuses on the Urbana track (I assumed Urbana was meant for the MD/PhD's since that's what they told us at the interview).
I'm actually from Peoria - but I chose Chicago, I think it is a much better campus overall than all the others. That's not to say that the other campuses are in total loss - to an extent it also depends on what you are interested in. For instance, I've heard Rockford is really good for Family Med. Peoria is actually pretty good for clinical experience - not as much variety as Chicago of course but there are three major medical centers there (St. Francis is tertiary care) that serve central Illinois. It's a decent small-mid size city and only a 2.5 hr drive from Chi-town. I actually worked at the Peoria Campus last summer - and it too is pretty much one building. Aesthetically, it is actually pretty pathetic. I think both Peoria and Rockford were built back in the 70's and you can tell. The library resources are really lacking. But actually it is starting to attract some decent research - like a pretty top notch cancer biology team that moved from MD Anderson in TX. Also, the administration there is working hard to bring even more research/money to the Peoria campus. Like I said before - the clinical experience would still be pretty decent there. Who knows there might be some benefits to a smaller campus and class -- i dunno maybe the ratios of students to a doc/resident are better during clerkships....
Sorry this post is a little messy...but I hope it helps a little.