LOL.... duh... I can figure the math out. JK... Anyway, from what I saw online, the 660w has a newer technology than 650w, or something like that. But really they didn't go too deep to have a further discussion.
The x660 is slightly upgraded; little different organization but performance and efficiency is so close to each other that it doesn't matter. The Corsair AX series is basically the original X and the slightly updated X series combined.
Basically, whichever is cheaper is the better deal since they're nearly identicle in everything. Efficiency , quality, and power output. Only difference is the price.. XD