IMO if you are considering a 7970 then you might as well get a 7950, it has great performance for 1080p gaming. (assuming you have a 1080p monitor) There are benchmarks that show that the performance increase is very small and i don't think it is worth it for $100+ more.
These are the benchmarks.http://www.anandtech.com/bench/Product/645?vs=508
AMD has great performance for your money while nVidia has PhysX and cuda. So it all boils down to whether or not you want the effects and cuda which is used for rendering and such, but i think not so much for gaming.