I don't know anything about greenboard quality. I assume it is all basically the same thing if it comes from a mainstream manufacturer.
The more cost effective and easier to work with backerboard is probably Denshield because it is also basically waterproof. The juncturers and the fastener penetrations need to be waterproofed after installed.
My all time favorite to date is WEDI but those guys have priced themselves out of my market. KERDI has a waterproof board coming soon, don't know if it is on the market in some areas or not.
