I know that the 6950 does have a performance edge over the 7850 while being priced almost the same. However, the 7850 does use appr. 70 w less than the the 6950 and is a better overclocker (so I hear). I have a couple of question:
1) How much money does the 70w difference in power draw save you over the course of a year (w/ at least 2 hrs of use per day), and do you think the savings are worth it?
2)If you oc both cards to their stability limits, how does their performance compare then?
3)How is PCIe 3.0 better than 2.0, and are the benefits/drawbacks worth it? Does 3.0 even have drawbacks?
The 6950 does not really have a performance edge over the 7850, especially when overclocked the 7850 gonna beat it pretty well.
I guess your savings all depend on how much you use your PC, obviously the longer the more you're gonna save. If you're gonna use the card 2-3 years very regularly, it's probably gonna be around 150-200$.
PCIe 3.0 does not really have any advantages for gaming, you shouldn't care about it too much, it has no drawbacks though(backwards compatible).
Overall the 7850 is the better card, especially with the drop in prices incoming.