Never had a problem with a Nvidia card, first ATI I had messed up, so I'd say my opinion might be based on a bad experience haha.
Well Ati It's the faster , Powerful , Better , fastest GPU on earth for the HD 6990
Nvidia Has Been Always Better Than ATI.
I've seen Nvidia and ATI.
Nvidia will destroy it. anytime anywhere
Don't listenen to those fanboys, they are both almost the some, some give better performace, some use less power or are smaller ect..
I prefer NVIDIA. Both are excellent manufacturers though. I just have more experience with NVIDIA though.