Voted for Radeon. Don't have a real "preference," but my last two cards (~5 years) have been ATI.
The (admittedly silly) reason I chose them was because, as someone who'd been out of the loop on what's "good" for years, I found their model numbering system really intuitive. It made it easy to judge which cards were worth learning more about. (Edit: To elaborate, if I see a mention of a specific model number, I instantly know where it sits in the hierarchy.)
Nvidia's naming scheme confused the hell out of me, so I never bothered to learn what the names actually mean.