According to Clubic and AnandTech, the winner is....

6990...for performance...But the GTX 590 is quieter and ok to overclock...But the GTX is too expensive vs 6990...But the 6990 is better suited for future technologies...But GTX would be better for actual games...

HAha! Anyway, a pair of GTX 570 in SLI is close in performance and cheaper...
And anyway , $700.00 is too much money !

Salute !