Well I couldn't answer before since I hadn't really decided, but I'm pretty sure that I'm going to do Chicago again this year. I loved it.
My other option was the towpath marathon in Akron, Ohio (on the same day so it was easy to start training), but a couple of things are making me lean towards Chicago. Firstly it's a big marathon and there are (as you know) masses of spectators giving support - the towpath is small and I'd be alone a lot I think. I don't believe I'm fast enough or experienced enough to enjoy the race so much if I'm trotting along by myself. Yet.
Secondly I have a friend in Chicago who's going to move to Florida soon and since I doubt I'll visit Chicago as frequently when he isn't there, I have a few restaurants that I need to get back to! Lastly, I've been watching my DVD of last year's marathon quite a lot lately and I'm feeling inspired!
I've also signed up for Richmond, VA, five weeks later. I have an internet buddy who did Pittsburgh this year and went out too fast and had to walk from 10 miles onwards. I might be nuts attempting these so close together, but I believe I can pace her well enough to get her to the finish in under her 6:22 and it would be fun. I hope! An experience anyway.
Then I have London next April... Send around the men in the white coats!