I had the same problem choosing between 7950 and 660ti and im really glad i went for the 7950.
As much flame as AMD gets with theyr quite-horrible cpu's then GPU's are a real sunshine from them.
I overclocked it to 1150MHz and paired with a 3570k @ 4.5GHz i get 55-60fps on ultra preset at all times in BF3 at 1080p.
My benchmarks beat a stock 670 and I think when it comes to 7970 vs 680 id say go for 7970 and overclock it (MSI afterburner will help, pm me if you need help) esp. looking at the price difference.
Best wishes
- cheti