Now despite the ~£25 price difference, in the Tom's hardware benchmarks of both cards you can see that game-game they are pretty much interchangeable. I will be using which ever card I get to run a 3D monitor at times for Films etc.
I would say go for the hd7850 if you can, as it beats the gtx 570 in certain benchmarks, and can match the gtx580 in others. It is also newer than the gtx560ti, uses and uses less power. So I would get the hd7850 or wait for nvidia to release the cards following the gtx680. I would not go for the older gtx560ti.