Hands down, nVidia has AMD beat card for card. However the 6950 is less than half the cost of the 580, and two 6950's outperform the 580, so for less than the cost of the 580 you can exceed the performance of the 580. If you needed higher performance of two 6950's, and only had two card slots, your only option would be two 580's, but seldom would someone need that, so it's pretty much left to the perfectionists to have 580's, and those who don't need the best of hte best would most likely go with the 6950 if cost were a factor.