Tried both. (BTW, this comment is 3d design/gaming biased, while the ATI may be a better card/value for non-hardcore gamers this is not directed at them)
Display for 2d is better on Radeon (non-hardcore gamers).
Performance is better on the GTS for 3d, in high res, in 32b color (hardcore gamers).
ATI had a huge following (they were always the biggest force in mainstream computing) even when they were the worst at delivering drivers, and not arguably, but definitely had the worse cards for gaming: 100% brand name.
3dfx and Nvidia were the only ones who could do anything for gaming. Too bad 3dfx is gone, the more competitors the merrier, but can?t rely on brand name forever.
Now, they (ATI) come out with a product, pit it against the GTS, yet released later, and still cant win (it is a 3d card first I think), yet get still get much support from loyal followers: surprised? There was another company that had a knack for playing on name Brand. Hmm, 3dfx had a bunch of hardcore followers that clung to the name; ATI is a company that was almost exclusively based on its brand name, sold a sh1tload of cards but sucked sh1t. Perfect place for the herd types to continue their support of blind Nvidia bashing. Radeon is good, ATI seems to finally be getting better, but lets stick to reality here.
Corporate loyalty blows; whoever has the best product for my needs will get my money. For now, for gaming and 3D, Nvidia can?t be beat. But as soon as someone does, I?ll be the first to say.