Radeon HD 7950 is faster by all means.
But has its own cons -
1) More energy requirement.
2) More noise.
3) Crossfire troubles.
4) Physx missing, etc.
At the moment, I would advise you to forget about both of them and go for GTX 760. It trumps both the cards and costs $250 which is cheaper than GTX 660 Ti and equal of HD 7950.
After the release of GTX 760, buying 660 Ti does not make sense.
Also, keep in mind 1 thing, future games might tend to become ATI inclined as PS4 and Xbox One both use ATI cards and developers would most probably keep that in mind.
So you basically have two choices, HD 7950 and GTX 760. Choose whichever you find cheaper.
P.S - When buying a GPU, CPU company does not matter. What matters is the CPU...