ATI would be the better choice if fps is all you are looking at. However, nVidia would be the no brainer choice if you care not only fps but image quality as well.
GTX260 vs. HD5870 in "Aliens Vs. Predator"
GTX260 in DirectX 9 delivers similar texture mapping and lighting as a HD5870 in DirectX 11 mode does (minus the DirectX 11 effects, of course).
I would even pick GTX460 over HD5870 if I was offered to pick one of them for free, because ATI sucks in 3D as you can see above.