HD 7970's are generally about 25% more expensive than their 7950 counterparts, but dont perform 25% better. I was wondering are their other things that make it worth getting the 7970 in terms of bang for your buck.
If money is a concern, then the 7950 is the better choice. If money is not a concern, then the 7970 is the better choice. It all comes down to how much you are willing to spend. This argument is also common in the GTX 670 vs GTX 680 threads.
The 7950 OC'ed can reach GTX 670 speeds and would also match reference HD 7970 speeds easily.