GTX670 is better than HD7950 in terms of performance alone and it's a more efficient GPU. Price to performance wise, it could be argue that the 7950 is better and that's not considering the great overclocking capability of the HD7950.
I'd recommend you get the GTX670 if you want better performance and a more efficient card (plus the PhysX and other Nvidia techs) or the HD7950 if you don't want to spend more and would consider overclocking the card.