nVidia cards are very easy to set up for near optimal performance and require only about 1 hour to fine tune a rig of 6 cards. A 6 card AMD rig will require over 6 hours of time to fine tune and get stable after flashing BIOS.

Having said that, my AMD cards are currently earning 5x the rate of their nVidia counterparts on a $ basis - they just excel when tweaked.

This last December run has seen nVidia outsell AMD for mining.