It’s true, without taking some precautions an Nvidia card will run Blender’s viewport noticeably slower (MUCH slower) than an AMD card.
Please read: http://www.blenderartists.org/forum/showthread.php?257323-What-everybody-needs-to-know-about-Blender-Viewport-performance&highlight=zalamander+viewport
I get great Opengl performance with my 7970 - however, no CUDA GPU rendering. That is why my second video card is a Nvidia 590, which suffices for most of my render tasks.
Nowadays Quadros and FireGL cards seem to perform drastically better again in most 3d apps - because the opengl drivers for workstation cards have been optimized (or rather, the consumer grade drivers have been “down graded”).
Having that said, I have not seen a real comparison of Blender’s opengl viewport running on consumer graphic cards and professional graphic cards. And it depends on the 3d software: for example, in Cinebench R15 I get 87 fps on my 7970 on my aging system with three (large) screens. A system with an [email protected] with a Quadro 4000 ($750 card) runs the same test at 65fps.
Too many factors really: it depends on cpu speed, number of cores, the software, whether custom workstation drivers are available for the 3d software you use (for workstation cards Max and Maya have severely optimized opengl drivers), and so on.
Check these scores, and decide for yourself:
Outdated, but still quite informative as to how the results can differ wildly depending on the hardware-software combo: