Besides certain games which prefer one brand to another, Most reviews indicate that at 2560 resolution, the 6990 wins. At 1680 and lower, the 590 often wins, however. At 1920... it's roughly a tie.
The 6990 seems like the better long term choice, as it doing better at higher resolutions seems to indicate it will handle more intensive memory games in the future. Also 2GB vs 1.5GB may help sustain the long-term viability.
This review really shows the phenomenon I mentioned:http://www.techpowerup.com/reviews/ASUS/GeForce_GTX_590/23.html
Although the 590 is ahead on average, keep in mind that average includes 1024x768, 1280x1024, which no one with a 590 or 6990 would ever use.