The 7970 is the strongest of all high end cards, it excels where kepler just cannot...
The memory bandwidth is much better with the 7900 series, and they are also a hell of a lot faster at direct compute, even faster than Fermi, albeit not a lot faster..
There is plus's for going with either vendor, if you like your PhysX, sadly only one choice... and CUDA is Nvidia only..
It depends on what you want.
For gaming only without anything else i have said, i would go 7950/7970.