I think it depends on what sort of experience you have had with a particular card, i had Nvidia for years, but for the last 10/12 months had nothing but grief, 2 x BFG 8800 Ultra's in SLI, i got so fed up i tried 2 x ASUS EAH 4870 X 2 cards, i now have a system that works great, if asked now i would say ATI, but before this i would have said Nvidia, so, it's pot luck, you can go on about this one does this or this one does that, but it's a coin flip as to which one is better, i don't think there is anything to chose between them, it's all PR and advertising BS.
Thats my 2 bobs worth.
As for Windows v Mac, i think that..........
HE HE, NO CHANCE. WE WILL BE HERE FOR EVER. Anyway, everybody knows Mac is better.
(couldn't resist it)