nVidia is probably the best option for performance discrete graphics on Linux, I already tried ATI and was deeply dissapointed, while nVidia drivers for my 4-5 year old laptop handle perfectly connecting to my external 1080p monitor my new (few months old) laptop with ATI graphics (A6 APU) totally fails to even drive the monitor over 1440x1024, even tough the ATI card has way more memory available to it.
nVidia propietary drivers work and are well tested, while ATI drivers sort of work most of the time but they are filled with glitches here and there.
Of course, not everything is pink and ponies on the nVidia side, the downside with nVidia is the little to no support to modern X stack and linux graphics standards, xrandr? fuck that use the nVidia panel or no dual monitor for you, KMS? hell no, its all or nothing, ATI is just only a tad better but still useless to me.
On the OSS side of the cards drivers the ATI drivers sort of work, KMS and basic stuff is solid but for modern cards you get mostly no acceleration for anything, which is just a waste of all that silicon, Noveau drivers are the same, basic stuff working but thats all, but Noveau developers have it harder as they have to reverse engenieer all, with no docs at all or help from manufacturer and they have been around less time than ATI devs.