The 480 is a tad bit faster, but already sucking down power and producing a decent amount of heat. For this and this alone I'd go with the 7870. Even if OC's are the same, you'll enjoy quieter gaming while keeping temps lower and using less power, all while getting pretty much the same gaming experience.
Also, the 7870 is fairly new, and has new drivers and such, which will get incremental performance boosts with updates, while the 480 is quite mature, and unlikely to see much more optimization through driver updates.
Hands down I suggest the 7870.
Edit> The costs savings with the 480 will be null after a few months use of gaming since you have to pay power. So unless its a decent difference, its a moot point at best. You could also twist it around and say eventually the 7870 will pay for itself in power savings vs the 480 after a while. Though thats more of a joke Edited by Fuell - 5/1/12 at 2:25pm