I'd put the extra cash in it. With some work the 6950 can be unlocked to become a 6970, plus you can overclock it. It's much more future-proof and will perform better on your 1080p display!
Actually, most of the 6950's sold now are not able to be unlocked. Only the very early models, like mine and some others that are mainly reference cards, could be unlocked. They have been laser cutting the shaders off of the cards, as opposed to just locking them.
You are correct about overclocking though, they do have a nice overclocking threshold.
EDIT: Forgot to add, the 6950 is a very good buy at that price.