Depends on your Resolution, and other things!
On smaller nvidia is better the higher you go the better AMD gets. Also the massive use of Shaders is suitable for AMD with there 4D shaders.
Crysis 1 and warhead are some of the games that really utilize them.
But if we talk DX11 you will tend to use nvidia they perform better at Tessellation.
Then there is 3D here rocks nvidia.
Then there is the possibility of Multimonitorsystems. Here its even. In my case cuz with AMD you can go with 3 Monitors and 1 CArd, while with nvidia you need 2 Cards in SLI cuz they only got 2 outputs per card. Further i think the nvidia surround scales better with the vids while AMD Eyefinity seems to cut here, but ingame is fine.
Then there is Multicards(SLI Crossfire), here nvidia is better in form of less noticeable micro bucking in fps. But the new HD6xxx series of AMD scales better up to an unbelievable 215% oO. though HD5XXX isnt as good as nvidia.
If you go for price/power i would go for a 2 x HD6950 Crossfire nearly the same as 1 GTX580 in price but 30% more gfx-power.
But as rumors say the gtx590 may be presented in two weeks also the HD6990 is still to come.
Nevertheless i think i depends on so much more: what your actual system is (not many boards feature both sli and crossfire), how many Monitors you got, do you want to play 3D - what type of( for me Anaglyph dosent work
while Polarisationfilters as in Cinemas work, 3D Vision too)
And what resolution you want to play.