Between the two it's hard to say, the 7870 will overclock better and provide slight higher frame-rates. However it's not going to produce higher IG settings compared to the 660.
7870 is both faster and overclocks better.
http://tpucdn.com/reviews/Zotac/GeForce_GTX_660/images/perfrel_1920.gif
7870 is 9% faster
http://www.computerbase.de/artikel/grafikkarten/2012/test-nvidia-geforce-gtx-660/4/
12% faster without FSAA, 7% faster with AA