I have always prefered ATI over nVIDIA. Gotta say, ATI seems to better as far as compatibility goes. the vary 1st GPU i bought was back in 2005 and it was a ATI Radeon 9550, that i bought a nVIDIA GeForce FX 5500 for a PCI-only computer than 2 more 9550s
For Macs, ATI/AMD can be finicky, for example, my 6790 isn't natively supported, but every Nvidia card I've put in it works fine.
For PC, just about anything works since you can download drivers from the manufacturer.