You can get 7950s for around 300 dollars, making them a bit cheaper on average then the 670. Both cards perform extremely well. In my experience, AMD's cards typically overclock a bit better (That is MY experience). I've got a good friend with a OC'd 670 and we compared in several benchmarks once. His 670 Overclocked beat my 7950 Overclocked by no more than 5-10 frames max in most situations. Some games, such as Tomb Raider, I've found to favor the 7950.
Bottom line you'll be satisfied either way. Both cards can max (Including 8X AA) 98% of the modern games out there at 1920x1080
Side note:
If you go with the GTX670 I would recommend the Gigabyte versions of the card and if you go with the 7950 I recommend the Sapphire and Gigabyte versions