I voted ATI, even though I currently have an nVidia card. I've used ATI for years with no real complaints or issues, but I recently upgraded my machine after seeing an amazing special offer on an nVidia card. It was a limited time offer and it beat all the other prices by such a margin I thought there had to be a catch, but there was no catch and the card works well. Trouble is I'm now working my way through nVidia's infamous drivers in search of one that doesn't have any bugs.
Anyway, super special offers aside I would say ATI all the way. Read the internet, all the tech sites are currently saying the same thing: buy ATI, no ifs, no buts. Here's a recent article for you: http://www.yougamers.com/articles/26436_the_winter_2009_guide_to_gaming_pc_specs/