If you mean what makes it have more performance than I'd say all of them. I know it doesn't answer the question but specs mean absolutely nothing. What you should be doing is checking benchmarks and other real world performance as there are much more variables a spec sheet can't show.
Yes, the answer is benchmarks, preferably game benchmarks and not synthetic ones. Just looking at specs can only even let experts give a rough estimate of a card's capabilities. If you aren't an expert you should just ignore the specs and look at actual performance.
That is an error in game programming and not something you should care about when deciding on a card.
The general image quality of both brands are very similar. In the latest article I have read on the subject I believe AMD cards in general had a slight edge but not to a degree that the average person would even notice.
So basically.. Image quality you will chose in the game, under the graphical settings. Wether its anti aliasin or increased world detail or whatever. No matter what graphics card you have, the image quality will not change. What will change with a newer and more powerful card, however, is how many frames of that chosen image quality it can render per second.
A Nvidia G 210 card can do crysis 2 and the higest possible settings much like a a GTX 590. The difference is that the geforce 200 will give you 1-2 frames per second, while the gtx590 will give you about 60 fps.