i was looking at building a gaming pc and i wanted to know witch one would be better for gaming
The difference between the two is not the most significant. It's about 3 to 10 frames. Varying on the game and settings. If you want to save some money, go with 670, if you want to make sure you can max all the new games coming out next year go with the 680.
With the price of the 680 recently dropping, I'd try to grab one for around $450. I personally have no regrets with my MSI 680 Lightning, and you can always SLI 680 to future proof when they drop further with the release of the 700 series.
Or for future proofing, pick up a 3gb 670 with a great custom cooler like the Asus DC2.