Radeon 7850 can be overclocked more, due to the unlocked voltage that comes with every AMD GPU.
Nvidia doesn't want their customers to tweak with the voltages. So most GPUs by Nvidia doesn't allow tweaking with voltages, there is some exceptions like the power edition by MSI.
So if you're an enthusiast that know how to get the most out of your GPU without damaging it, then the Radeon 7850 would be the best deal. If you're like most people that leaves their GPU at stock speed or just overclock a little, then the GTX 660 would be the better option.
And I guess your no enthusiast since you're asking this question, so the GTX 660 would be the better option.