nVidia at the moment is a bit of a risk, as there is a whole raft of issues going on with nVidia hardware. If their drivers work, they will likely give better performance and features than AMDs options, and so long as the hardware holds up. Read up on nVidia bumps issue for more information.
However... AMD have an open-source strategy and an official open-source driver stack. Their open-source drivers are good, and getting better, but are not feature complete yet, nor really competitive for performance in 3D games. I'm not aware of any major issues surrounding AMD graphics, except the general complaints that the FOSS drivers aren't yet good enough. YMMV, RobbieAB
