Either of the AMD or Nvidia cards will do well. I have had both Nvidia and ATI cards and they are competitive.
The GTX 560 ti scales well in SLI, so if you add a second one later it will give you about 190% of a single card. The AMD will give you about the same.
The advantage of the AMD 6950 is that it, often, can be flashed to a 6970. If you can flash it successfully then you have a bargain.
I have never OC's an AMD, but the reports are that they OC well. I can personally attest to the fact that the Nvidia products are designed for OC'ing.
I am running a GTX 470 modded out with the Zalman 3000VF cooler and OC'ing it to 800/1600/1600. It runs cool and I still have room to OC more. The problem with the GTX 470 is its RAM which is very sensitive. Increasing the GPU and shader clocks is simple and easy, but the memory is touchy.
My personal preference would favor the Nvidia, but the AMD is a respectable choice as well.