950 MHz for ALUs/TMUs/ROPs(1792/112/32)
= 3404.8 GFlops/106.4 GTexels/30.4 GPixels
5 GHz 384-bit GDDR5
= 240 GB/s
1111 MHz for ALUs/TMUs/ROPs(1344/112/24)
= 2986.368 GFlops/124.432 GTexels/26.664 GPixels
6.608 GHz 192-bit GDDR5
= 158.592 GB/s
In my conclusion it would appear that the 660 Ti has faster timings(Renders the scene faster) and does more efficient texel work(Can map the textures faster).
--> Higher clocks = faster rendering. <--
Games don't use the ALUs, the ROPs, and the RAM efficiently on the PC, so more Hz means more power even if you have significantly less units.
Games(+HPC with CUDA): Nvidia <--unless AMD is cheaper for the same performance.
High Performance Computing(Not with CUDA): AMD