I'll list the GPUs that you should look at against the other brand;
7970GHz Edition vs. GTX 680
7970 vs. GTX 670
7950 vs. GTX 660ti
7870 vs. GTX 660
7850 vs. GTX 650ti
7770 vs. GTX 650
7750 vs. GT 640
If you get the 7950, you can get GTX 660ti performance AND the 7950 overclocks A LOT more than what a GTX 660ti could. You could also get a 7870 and overclock it to reach GTX 660ti levels.
So 7950 most likely.
To see what should go against what, look at the prices. The price for a GTX 660ti is around $300 and that is what it costs for a 7950, so you should be comparing a 660ti and a 7950.
More like:
7970 vs. 680
7950 vs. 670
7870 vs. 660ti
7850 vs. 660
7770 vs. 650ti
7750 vs 650
Radeon HD vs GTX