If you intend to play the Batman games, then the GTX 560 Ti will be essential in order to enable the Physx features. Physx is not supported by the 6950.
Quote:
"Gamers looking to upgrade for Batman: Arkham City are probably best served by waiting for the game to be patched. But for those who just can't wait, for whatever reason, an NVIDIA GPU will give you the best gameplay experience in this game right now and down the road. Even when DX11 mode is fixed, AMD video cards still won't support PhysX in hardware. As always, gamers with giant monitors should be considering high-end video cards. An NVIDIA GeForce GTX 580 will provide the best gameplay experience Arkham City is currently able to deliver. For gamers with a tighter budget, a GeForce GTX 570 is a fantastic performer and will allow the highest PhysX setting at high resolutions. For gamers with smaller displays, the GeForce GTX 560 Ti is still a fantastic option."
http://www.hardocp.com/article/2011/12/05/batman_arkham_city_gameplay_performance_iq_review/8
Skyrim has also been playing much better on Nvidia cards than AMD cards:
"AMD video cards have some performance issues in Skyrim, and we do hope that AMD can improve this with driver updates. It would be difficult to recommend an AMD video card to any gamer looking to upgrade his or her computer in order to play Skyrim. For this game at least, NVIDIA-GPU based current generation video cards is where it's at, whether you are packing a 30" monitor running at 2560x1600, a 1080p panel, or a multi-monitor gaming solution, they are simply faster."
"Gamers looking to upgrade their video cards for Skyrim should look at NVIDIA's offerings. For high-end enthusiasts, the single and dual NVIDIA GeForce GTX 580 solutions both rocked the game out of the park, and the GeForce GTX 570 was close behind. Gamers with a more modest budget should consider the GeForce GTX 560 Ti as a solid option for 24" and smaller monitors."
http://www.hardocp.com/article/2011/11/21/elder_scrolls_v_skyrim_performance_iq_review/7