Its not so much the cards, but the drivers. CCC and Nvidia control panel have options to change the color settings and what not. Default, Nvidia have it brighter and more pop, kind of like bestbuy and other stores adjusting the settings on their TVs. ATI goes for a more life like image. If you play around with those settings, and your monitor/TV settings with a proper calibration tool, you shouldnt see any difference at all.

Not sure what happened to the pics, but here,
http://clubnboc.com/forum/index.php?topic=98.0
Ether did a comparison, and there wasnt much difference.

Whatever it is, it's like night and day on my screens. :)