I trained at UofC... so I'm a little biased. But I think those who rotated through both departments would agree that UofC's southside patients are more sick and the level of autonomy does not compare (good for some people, but can be a disaster for others). At UofC you will be the patient's sole physician with a senior resident guiding you from day #1. Not to say that Northwestern doesn't offer sick patient's, but when I went through the ER as a med student, I noticed a difference in that regard.
If you like learning by the book, lectures, and strict academics... go to northwestern. They will guide you more clinically... The attendings oversee the ER much more closely.
If you like learning by trial and error in the department, learning by fire, and thrive off independence and being autonomous... UofC is your program. Nobody will baby you, and nobody will hold your hand...
Both are good programs, but very different. If you are unsure, spend a day in each department with the senior resident. That should answer any questions about which program is right for you...