AMD GPU's have no CUDA support, if someone wants to do some computing on it, usually OpenCL is the way to go.
Last I heard CUDA will outperform OpenCL on nVidia vs AMD at the same price point generally as a result of CUDA being in house and closer to hardware. As a result if you just care about compute performance you would go nVidia. If AMD offered their own Mantle based compute offering it would probably shift the other way.