Having just done an upgrade from a Radeon HD3870 to a Nvidia GTX660Ti, I must say that AMD drivers are fairly good.
I think AMD are better at Linux integration as in RandR and start up modes where my Nvidia driver starts Kubuntu with the screen orientation wrong for my dual monitor setup. I also have been getting flash video overlay problems which I didn't get on AMD/ATI.
Nvidia seems to be better for Wine games, as I now don't get missing textures in Everquest, that I was experiencing before and is a known problem with AMD/ATI.
I wouldn't have a problem getting another ATI if this Nvidia card I have now, dies. I worry as Nvidia cards are known to have burn out problems with past releases.