GTX670 Wins unless you both luck out on a 7950 chip with high overclock capabilities and are willing to overclock, a lot.
It's a gamble, you may end up with a gimped 7950 that can't even outdo a 660ti. Plus by overclocking you void any warranty. Unless money is a concern I'd recommend the GTX670.
The GTX 670 is the faster card.
I do have to comment on your choice of words, however, when describing the 7950.
It is not lucking out to get a 7950 that overclocks well, as most tend to do. It is actually extremely unlucky if you get one that doesn't like to oc much higher.
Anyways, your point remains true, the GTX 670 is a faster card than the 7950 on the whole.
If you are considering a 7950 then this Sapphire Dual Fan X 7950 is a well priced choice at 299.99 with $20 rebate taking it down to 279.99
I have seen two of these cards in friends builds and each one has oc'd to 1150 on the core with being pushed too hard.