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IF the 6970 is basically two 6870's, then the 6970 will best 580 on average over multiple titles.
Will probably end up wrong of course lol.
On the highest end card? price is something you worry about with a mid-range card.
If the 6970 was faster than the 580 why would they price it 100 cheaper than a 580? They could make more money if they pushed the price up and they can because it would be faster than the 580 (But thats if its faster than the 580).
Im expecting btoh cards to be similiar in raw grunt maybe ~5% off for either one but the 580 will be better at Tesselation/DX11 titles and the performance with 3D will be much much better on with the 580 and 3D is the future in gaming so I think the 580 will be the better card.
If the 6970 was faster than the 580 why would they price it 100 cheaper than a 580? They could make more money if they pushed the price up and they can because it would be faster than the 580 (But thats if its faster than the 580).

It doesn't quite work like that. The GTX480 is faster than a 5870, but I'm sure you can guess which one sold the best, right? Lower priced cards sell better, the top top end sells much worse than the mid-range simply because the vast majority of people are more willing to spend <£150 than >£300