With the GTX 680 you also get extras that the 7970 can't match, like PhysX, Adaptive Vsync, Geforce Experience, Ambient Occlusion, Turbo Boost, FXAA, TXAA. Just looking at the relative performance doesn't give a full picture.
The gtx 680 will run better on some games just like any comparable cards, but like I said before it just depends on what you need. Looking at other specs the GTX670 will give you a lot more back for your buck its $100 less than a 680 for me and is only like 10% slower and can be overclocked to match and possibly surpass the 680.
Heres a good thread comparing the two http://www.tomshardware.com/forum/355726-33-radeon-7970-balck-edition