They are cheaper because they are slower and have less features. Like an economy car. If they were faster and had more features, they could and certainly would sell them for more money. It's not AMD.org
Yea, you are "exactly" right. They "are" slower. At lower resolutions. And just about = at higher resolutions with higher settings.
Cheaper by almost $130 and that is how much money you save by purchasing a 6970. Although you lose a "whopping" 10% performance.
How do they have less features ? Nvidia doesn't have Powertune. And refresh my memory, for 3D, how many NV cards do you need ? One or two ?
For Eyefinity, you need "one" card.
Cuda and Physx isn't features. To me anyway. You barely hear anything about Physx now. And to me, Cuda is not important. And tessallation, in a few years when it matters, maybe. But not now.
These arguments can be spun around all day long.
But in the end....the GTX580 is "green" with envy.