I own both 580 SLIs and 6970 xfire in 2 PC's and I can tell you that there are many, many games where override settings are outright ignored with nvidia drivers. I have to wonder if nvidia gets inflated benchmark scores in reviews because of this? Games I personally tested to find this recently were Dead Space 1, Dead Space 2, and Dead Island. Now AMD certainly has its faults and I prefer the 580's overall, but the one good thing about them is that it ALWAYS obeys override settings in CCC. If you enable SSAA in CCC it will work, you can use it in Dead Space and the game looks -fantastic-. Now on nvidia you can't even use SSAA -- and before you say it, SS transparency is a joke and is NOT the same thing is SSAA. Due to this AMD has better image quality generally speaking, because override settings ALWAYS WORK. With nvidia, its a crapshoot. Many games are forced to completely ignore "override" settings in 280+ drivers.
From what i've found nvidia drivers 280+ ignore "override" settings in roughly 65% of games. Of that 65%, you can fix 10% of them in nvidia inspector. You can plainly see in nvidia inspector that a great number of games are forced to ignore override settings - look for yourself. There is a setting flagged as "Treat override settings as use application preference". And even changing that in nvidia inspector does nothing.
On the flip side of the coin, nvidia has a better games profiling program , by far. And nvidia has been quick to push out SLI profiles for new games recently, such as bf3 and skyrim. I prefer playing bf3 on my 580 setup. It sure would be nice though if override settings worked without exception, though. And SSAA would be awfully nice to have.....Dead Island has so many jaggies it makes my eyes bleed. Speaking of which , Dead Island looks better on the 6970 crossfire because you can override AA settings. With the 580's, override settings are ignored.