Which is better? SAPPHIRE HD 7970 GHz Edition 6GB vs hd 7990 6 gb?

MOTHERBOARD: Gigabyte GA-970A-D3, CPU: AMD FX 8150 Black Edition, VGA: Gigabyte HD 6970 2GB, RAM: Kingston Hyper X Blue 2x4Gb 1600Mhz

Dual-GPU cards always list the combined memory, which means a 6GB 7990 is the equivalent of two 3GB 7970's (more or less, depending on the clock speeds in use).  So a "6GB" 7990 has twice the rendering power and half the actual memory as a 6GB 7970.

The cases were 6GB per GPU would be better than 3GB per GPU are few and far between.  If you don't have a 30" monitor or several smaller monitors in an Eyefinity setup, you won't encounter any of those cases.