I'd wait a few weeks if you can. There's supposedly a Nexus event near the end of October, might really change the sort of options out there.
I'd also suggest you'd get near double that selling an iPhone privately than trading it in.
The next Nexus seems to be coming from LG, a brand that I am not too fond of. And will they carry it on AT&T? From my knowledge, the recent Nexus products are not carried on AT&T (the last one, I guess is the Samsung Nexus). I have to stick with AT&T because I am already on a 2-yr contract with them.
I have been eyeing the Galaxy S3 for a long time now, and after seeing one of my friend's S3 last week, the eager-ness to get one myself has surely increased more. And my iPhone 4S is a locked one (to AT&T). Do you still think I can get more than what Best Buy offers me for the trade-in?