The HD 6850 and GTX 460 1GB are equal in terms of performance, the one card scores 1-2 fps more on one game, the other one 1-2 fps more on other game. The GTX 460's stay cool and are more overclockable but the HD 6850 is 20-30 $ cheaper. (i'm talking 'bout 1GB GTX460's, the 768MB versions are 10-20$ cheaper than the HD 6850 but can be a lot slower @ 1080p with AA on)
So this might be a personal preference of brands after all. And you have the HD 6870 @ 220 $ (the GTX 460's 1GB starts @ 210$), which might also be a option for you.