As the 7950 keeps up with the GTX680 when overclocked (check
toms review of the Club 3D 7950 Royal King) , I'd go for the 7950
As said above, unless you have a specific requirement for CUDA or Nvidia the 7950 is pretty amazing for the price when overclocked.
Also most CUDA apps now support OpenCL (eg: adobe photoshop, sony vegas, powerdvd, gimp)