Well, I signed up for AT&T GSM as a charter member. The reception was absolutely horrific. I have seen all the ads about how great their reception is now. Unless it has changed dramatically in the past 3 months (which is def. possible) I would stay away from their GSM services.
Then again, I had Sprint 7 years ago and it was terrible. I currently have Sprint and have not had a single dropped call or fuzzy reception since I started my contract (3 months ago.) Times change I guess. I am located in Los Angeles if that makes any difference. Good luck, and let us know how the reception is.