Even steven, hence similar pricing. Both over clocked can equal STOCK 580 performance. Bear in mind the 580 can be heavily over clocked and pulls away in SLI or not either way. Benchmarks by reviewers don't take into consideration over clocking. So that theory that two 6970's in SLI can almost match performance of 580's in SLI is not entirely acurate.
GTX 570's do come in 2.5GB VRAM where on multiple monitors will pull away from the 6970 anyway. Since your getting two eventually three monitors isn't an issue. So that's another scenario debunked.
Back to your choices. Here's a logical way to look at two cards with same performance. The answer actually lies with the purchaser who should ask themselves this question.
IF your looking into possible multiple monitors and IF your choosing between the 2GB vs the 1.25GB VRAM cards, then go AMD. IF your looking at the 2GB vs the 2.5GB VRAM then go Nvidia.
IF your looking into possible 3D gaming and watching 3D blu-ray movies then hands down go Nvidia. On a single monitor there's very marginal difference between VRAM as anything under 1920x1080p isn't utilizing it like multiple monitors.
IF your not looking into either option, then flip a coin. Seriously go with what you like or like to try instead.
Hey you can try either option for 30 days, most good retailers will take it back for refund or at the very least store credit. Either way you decide your going to be happy and you won't need to look back with remorse or doubt.
Hope this little analogy helps. Naturally peoples choices varies and in some cases is better than others. Hence why it's really up to your probable usage for the card that must be taken into consideration in making the decision that suits you best.
I remember reading your post, Corsair is hands down right up there with EVGA for customer warranty and customer support. Awesome how they stand behind their products. Good experiences goes a long way.
Good luck and make sure you finally update this thread with your decision as I'm always curious to find out the results.