If you already have the NEXT plan, I can't really see how you would benefit from also having the Apple Upgrade plan (especially if you're sticking with AT&T anyways). This Apple Upgrade plan only seems relevant to people who are not under a contract but do not want to foot the big up front cost of a new device that is out of contract, and may switch carriers.
I'll be totally honest, I heavily considered getting a subsidized device this go 'round because I don't plan to leave AT&T as I'm very happy with their service. The only thing that stopped me was the fact that me and my wife might be moving temporarily to California next spring for her work, so we might have to change carriers if AT&T isn't as strong a service out there vs. here...otherwise I'd take the cheaper contracted cost for this new phone and save some cash since I'm not going anywhere anyways.
Logic tells me that if you're with AT&T NEXT right now already, it will be cheaper for you to just stick with them over the period of time...instead of either paying for two upgrade plans (NEXT and Apple Upgrade) or paying off your NEXT device outright and keeping the other.