Honestly it depends on which game you are playing. Nvidia cards play better on some games and AMD on others. Also, traditionally Nvidia cards fare better in SLI than crossfire, but AMD turned heads with the 6000 series as the performance of Crossfire has matched or beaten SLI configurations.
Personally, I would go with the 6950's as they can be flashed to 6970 specs rather easily. But, you should buy the cards for the game you are playing. Look up some reviews of the 6950 vs 560ti.. Then make a decision.
As far as longevity is concerned, EVGA has a really nice warranty and I have had nothing but good luck with them. If you decide to go with AMD I would personally use Asus.
Well rounding everything together vs wise. NVIDIA fairs better with Drivers. AMD is best at Price and performance best deal. But i still choose NVIDIA because they have good drivers, SLI can be more Stable and I have a GTX 560 ti and havnt had any issues