The ATI/AMD situation is quite complex. Many users have no problem - but that may also be because they don't really stress the GPU. And there are also people who have problems with NVIDIA. Sometimes the nvidia driver installation fails, and you need to run sudo nvidia-xconfig in a console to get your system back to a working state. For both manufacturers, there are a lot of users who have no problems and some users who have problems.
However, generally, NVIDIA is working the best. For AMD the performance is often a bit lower, however 20% is really much.
Have you already tried using different driver versions? That may also solve problems for AMD. Catalyst 12.4 seems to be working pretty well on most hardware.
Registered Linux User #528502
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