IMHO GTX 580 is still a better buy if your not dirty poor and compromising to save few bucks.

Game Compatibility
Nvidia has generally better (with all the cash they dumb in game companies TWIMTBP titles ect) yes such company bias is bad but it is what it is and we have to live with it, it can be a bonus if your on such camp nvidia does put more effort in getting certain game features working, you don't see AMD putting such efforts.

Sponsored Games
As such sponsored games by nvidia eg. Lost Planet 2, Hawx 2 ect. perform substantially better on geforce line. (it might be a bit to early to make this claim as we don't have enough direct 580/480 comparisons only 5870/6970 but looking at LP2 a stock 480 is substantially faster)

Single GPU Preference
Taking above into account Single GPU performance wise GTX 580 is a fastest you can get. (A bit premature i know but my claim seem logical)

Extras
PhysX (hopefully it will die and get replaced by something better but for now it is free Candy if you have Geforce no such perk from AMD side)

CUDA (not really relevant to games but those who use programs who support it, know its a nice bonus to have)

3D gaming (currently the only solid platform from nvidia, AMD response is pathetic with so little effort put into it)

Mutual Abilitys
Multi Display gaming (Need SLI for 3 monitor gaming, this one AMD is one step ahead sorta still need crossfire for acceptable FPS especially with more then 3 monitors)

Both can do SLI/CF (nothing to see here)

*puts on anti troll ear plugs*