The Green Bay Marthon in Wisconsin is the flatest course I have ever run and the road quality is better than Chicago. The difference between the 2 races is simple for me. Chicago boasts a much larger crowd which helps me dig a little deeper when I need it. If you are the kind of runner that prefers few spectators I would suggest Green Bay. The start and finish has a larger crowd but after the half mark you are pretty much on your own.
Like Chicago you can never tell what the weather is going to be for this race as it is in mid-may in Wisconsin. It was 78 degrees the day before the race in 07' and 28 degrees at race time. Good luck in finding the flatest course.
PS - Don't let the course records (2:17 - GB vs 2:05 CHI) fool you. Green Bay doesn't boast nearly the same prize money($2,500-GB VS $75,000 CHI $100,000 additional can be won if a course record is set) compared to so the best in the world never show.