Well there's really nothing to argue about when the numbers are available. For current generation DX11 cards, that is what people are buying and using as of January, based upon what's on gamer's boxes when they hit Steam's servers:
Card .......................January Market Share
NVIDIA GeForce GTX 560 9.47%
NVIDIA GeForce GTX 570 4.02%
ATI Radeon HD 6950 4.08%
NVIDIA GeForce GTX 580 3.69%
ATI Radeon HD 6970 1.72%
NVIDIA GeForce GTX 550 2.96%
ATI Radeon HD 6970 3.43%
Before the green team fanboi's get all excited, the post popular DX11 card, albeit last generation, is the 5770 with 13.23 % and the 460 w/ 10.0%.....the 5870 and 5850 also did very well compared the nVidia's 4xx offerings.
So .... if ya mean "today what are most people buying ?", It's about 2 to 1 for nVidia .... Last year, it was ATI/AMD that had the edge.
Peronally, in my builds......in the under $200 segment, I go red primarily using the 6870 ...... over $200, I go green, primarily using 1 or 2 of the factory OC'd 560 Ti's.