zotac 660 ti amp vs Powercolor 7870

Solution
The GTX 660 Ti is faster. You'll also get Nvidia driver support and the ability to enable PhysX, Adaptive VSync, DSR, TXAA, FXAA, HBAO+, etc.

http://www.techpowerup.com/reviews/Zotac/GeForce_GTX_660_Ti_Amp_Edition/28.html
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Very generally, the GTX 660 Ti uses a bit less power, and runs most games a bit faster than the HD 7870. Otherwise for general purpose work they perform similarly.

Overall the GTX 660 Ti seems to have the edge in performance (particularly for gaming), so this would tend to be the best card of the two.