There is no difference in image quality based on the brand of card you buy. The days of 2D image quality being a function of particular graphics cards are long gone for most people. When CRTs were popular, it was something that mattered as the quality of the RAMDAC controlled the quality of the analog VGA output. Thus different cards, even with the same chipset, could potentially produce different 2D image quality.
However LCDs and DVI have changed all that. The signal is fully digital from the OS, to the card, to the monitor right up until the individual sub pixels are finally lit up. This removes any quality differences from card outputs, since they are outputting a completely accurate digital signal. The only time they change anything is when instructed to by a change to their LUTs from the user, OS, application, and so on.
What that all means is the conclusion is pretty clear: There are many good reasons to buy team green or team red, but 2D image quality is not one of them.
They are precisely the same.
Color Test