The 760 is a pretty big step down from the 7970 in a variety of areas no matter which way you cut it. Not sure why it's being suggested. A 770 over a 7970 I could understand, however.
both are the same price (free Batman Origins with GTX which I was planning to buy) and according to some sites the 670 is supposed to perform better and Nvidia in general is better for WoW in particular. I could just go for GTX 770 but would like to keep the price down.
If you dont intend to overclock, I think the 670 performs a bit better at stock clocks compared to the 7970.
When you do overclock, I think they are pretty even. They both have games where they outperform the other, but WoW does favor Nvidia (altho both are more than enough for wow )
You also get 3 games with the 7970. So pick one you like the most, you cant go wrong with either.