For HD h264 and VC1, all you'll get is that marginal cpu difference - neither has anywhere near enough power to do processing like spatial-temporal deinterlacing or bad edit detection (despite NVidia's own spec sheet claiming they can).
For mpeg2, the only real difference is HD mpeg2 bad edit detection, and that still doesn't work properly - you're better using dscaler5 than Purevideo.
For mpeg2, the only real difference is HD mpeg2 bad edit detection, and that still doesn't work properly - you're better using dscaler5 than Purevideo.