If you plan on overclocking your GPU, neither, get a 480. Overclocked you are looking at performance over and above that of a 580, but with a lower price tag, even if it does create more heat. If you are keeping it stock, I would take a 570, however, do not rule out a 6950, it comes in cheaper than the 570, but performance wise isn't a million miles off, and if you unlock it to a 6970, it will be on par or above the 570
Wow really? Even with the unlocked CUDA cores? :S That's pretty surprising. I would have thought with the heat issues the 4XX series has that overclocking that much wouldn't be feasible.
I'll second your comments on the 6950 though, excellent card for the price, not to mention the high possibility of getting a 6970 out of it.