Honestly, I think you're better off paying it off fully and selling it on Ebay. I bet you could get way more than what AT&T will give you. I don't know what the number is, but you have to have a certain amount paid off before being able to trade it in.
Don't completely discount the convenience factor of being able to just walk into the at&t store and swap out one Next phone for another every year (I'm on the 12 month plan). I don't mind paying to be able to avoid messing around with third-party reseller sites and stuff. It's not for everyone, surely, but that is certainly a factor for me and my decision to use Next.