Normally I would go with the gtx 560, it has slightly better performance than a hd6870. BUT the gtx 560 that you chose is already overclocked, so it will not only be more or less on par with a 6870, but will be better. Also with nvidia you have CUDA, and Phys x, plus generally drivers are better. Another advantage is that nvidia does better with tessellation. Go for the gtx560. What system do you have? What resolution will you be playing at?
Hope this helps