GPU pw cracking question

[Mopey]

Hi all, first question in here so Hi.

Building a medium-budget gaming/learning to use Kali/Backtrack machine and wondered about something.

What is the most important feature for GPU cracking - just plain clock speed? memory clock? CUDA vs OpenCL?

My budget for a GPU is £200 and want the most gaming and passwork cracking bang for my buck. At that price range GTX660Ti and HD7870 seem to be about the most rockstar type cards available. Now I've got little preference between brands, so it comes down to which will be better for doing stuff with

Any thoughts or opinions welcome

Thanks

[Mopey]

Actually a slight amendment to the above...

Nvidia GTX660Ti is £180
AMD HD 7950 is £220

Now, as far as I can tell the Nvidia is a faster clock and memory, but the AMD has more processing units (about 33% more). 7870 while in the price band is a slower clock and fewer processing unit, so whichever is more important it's going to lag behind the other two and isn't sufficiently cheaper to think about getting two