Ok, Nvidia has the pixel shading model 3.0 and comes with a lot on video color and settings. ATI lacks on this.
ATI has crossfire, but it looks messy as it is on the exterior. Nvidia is interior.
ATI has some nice cores, while some Nvidia don't.
So, if you look at the three, if you want PURE performance, get ATI. If you want neat things, get Nvidia. If you are video editing, get Nvidia Wuadro or 3D labs. If you play music and stuff, get either both as a budget card. If you like nice clarity, get Nvidia.