From what each company normally does with their special versions.....I usually look at Asus and MSI.....and I'd be buying a factory overclocked model as they usually come with upgraded coolers and more robust VRMs. Here's a summary of the 6950 and 560 Ti in both reference and factory OC'd models
Guru3D uses the following games in their test suite, COD-MW, Bad Company 2, Dirt 2, Far Cry 2, Metro 2033, Dawn of Discovery, Crysis Warhead. Total fps (summing fps in each game @ 1920 x 1200) for the various options in parenthesis (single card / SL or CF) are tabulated below along with their cost in dollars per frame single card - CF or SLI:
$ 220.00 6950 (479/751) $ 0.46 - $ 0.59
$ 240.00 6950 Frozr OC (484/759) $ 0.50 - $ 0.63
$ 205.00 560 Ti (455/792) $ 0.45 - $ 0.52
$ 215.00 560 Ti - 900 Mhz (495/862) $ 0.43 - $ 0.50
In a single card configuration it's a tossup at 484 to 495 fps but if considering SLI / CF, I'd get the factory OC'd 560 Ti for the 103 fps advantage