I'd say go with the Nvidia, "performance" be dammed-
For the last couple of years, I'd used a ti4400 Golden Sample. When it crapped out, I slapped a Nvidia Quattro in, and used that for a few months. With all of the negativity concerning the Nvidia line and DX9 games, when it was time for my upgrade, I decided to try out the ATI line.
Now, I went thru and used a driver cleaner, took out all of the Nvidia vid drivers and files, etc etc that "might" have caused problems with the ATI 9800Pro I was trying to install. Great. Install went like a breeze. HOWEVER. Ever since the ATI has been in my PC, I've had random crashes, lockups, etc etc playing Call of Duty (the only game I've been playing since I installed the 9800 Pro) At first I thought it might just be because I had everything cranked up on it, so I turned it all back to default...however just crashed to the desktop again about 10 mins ago.
Soooo. In a nutshell, Nvidia is STILL the rocksolid choice when it comes to drivers. Irregardless of what the fanboys say, ATI is NOT the end all be all of the video card world, and IMO, their drivers STILL SUCK as bad as always. This $300 pig goes back soon, and it will be my final stray from a solid product line...ATI no more!