I'm going back and forth, have a 3GS and 4, 3GS is jailbroken and love the way I have it setup right now. May upgrade to either the 4S or SGII since I don't know of any other new Android phones coming to AT&T. And sticking with them since we have grandfathered unlimited on all lines. The only thing I don't like about the Droids is a lot of the apps are unpolished and glitchy vs the similar or same offerings on iOS. But there's no jailbreak for the 4S yet and not sure when it'll be available. The new notification system is pretty slick when integrated with SBS and so customizable, a lot of new widgets are being developed for it on Cydia.
The 4S is on back order right now, called all the stores around me and I'd have to order now and wait 2-3+ weeks. And I have a feeling the GSII will be around $50 or so as a special towards the end of this month like others have mentioned at Radio Shack and maybe other outlets which would be a hard deal to pass up.