The 670 and 7970 perform basically the same at stock up to 1080p resolution, trading blows depending on the game. The 7970 most likely will OC better and it will murder the 670 at high resolution (and has way more compute power), the 670 gives you CUDA , PhysX, TXAA, FXAA, ADap. Vsync etc and lower power draw. Both are solid buys.If you still can't decide which card to take try looking at the games that you play or specific features that you want.If cost really matter to you then you might want to go with whatever is cheaper to you.
I don't believe they sell pre GTX680 7970s anymore, which means a current model 7970GHz edition will murder the GTX670 in about anything, if you're getting one of the old stock 7970s, then it's more debateable but I'd go with the 7970 still because of the better frames at high resolutions and more overall power. PhysX is a marketing trick IMO, all those technologies I wouldn't pay too much attention to.