I'd say the 9800GTX+ wins in more of the tests I've seen, and it wins in 3DMark. The ones where it loses, the difference is a couple FPS.
Overall, the performance difference is negligible. What is really turning me toward nVidia is the drivers. I'm sick of the Radeon crapware. Why should I have to install the .NET framework just for my drivers to run properly? I find the Catalyst drivers to just be much less stable than what I've experienced with Forceware, which is 100% stability.
EDIT: Oh, and I forgot to mention that the price difference is only $3. $170 for a Palit 4850 (what is Palit...) versus $173 for an overclocked 9800GTX+. Clearly the 9800GTX+ wins.