You’re posting this in the FCP forum. It’s a well-known fact that the GPU-dependent apps in Final Cut Studio perform best on ATI cards due to Apple’s driver optimization. It’s likely that an ATI Radeon HD 4870 will outperform both of the nVidia cards in Motion and Color.
Historically, Apple’s nVidia drivers have lagged far behind nVidia’s WIndows drivers, to the extent that Quadro cards on the Mac were a total waste of money unless you required stereo output. nVidia recently announced that it will be taking over writing Quadro drivers for the Mac, so it’s anyone’s guess what the future will bring.
More info here:
https://arstechnica.com/apple/news/2009/12/a-second-look-at-the-nvidia-quadro-fx-4800-mac-edition.ars
Excerpt:
“This should be the perfect card for me: a professional 3D artist who often makes Blu-ray movies from HV20 footage. But the benchmarks speak for themselves—the framerate loss in Maya and Mudbox relative to the Geforce GTX 285 would simply hurt my day-to-day 3D work.”