The 560 is more expensive, and about as good. They both overclock well (560 is better) but with the 6950 you also have the option to try the 6970 unlock, which would be significantly better.
Another important thing to keep in mind is availability. The 6950s are very popular and as such it would be easy to get another or follow through on an RMA, however the 448 core GTX 560ti is limited edition so there won't be many to be had in the future and the price will go up instead of down.
noahscorp, you're getting Ghz and GB mixed up. First is frequency, second is number of bytes. More to the point, there are now games that can use the extra VRAM and since OP hasn't mentioned his monitor resolution (you should tell us, OP) we can't be sure of what he may need.