Generally speaking the AMD Radeons have better color and more stable driver releases and the Nvidia's have better performance and more frequent driver updates.
Have had mostly Radeons over the years, my old box has a 5870 in it, works fine. My newer box has a GTX680 and the laptop has a GeForce GT 650M both work great.
The advantages of Nvidia in professional applications are with the Cuda implementation and I have the advantage of using the OS or Nvidia's web driver on the Mac, so I can switch easily if their web driver does something funky.