That's not an opinion as much as it is a decision based on my past experiences with both cards.
But there IS a fact of the matter, and that is that nVidia cards simply DO outperform ATI cards. If you were to make two identical rigs, same RAM, same Motherboard, and CPU, same EVERYTHING. And the only difference was the card, the newest nVidia vs the newest ATI then the nVidia would win. I know this because there's people who have DONE tests like this. With the latest generation of cards nVidia wins.