Both cards are disappointing but I would choose the 6990 at this point because of the overclocking danger that might be on the gtx 590. Both are fast enough for current games but none the less, I would say the 6990 is slightly faster. BUtthe only reason I would get get a 6990 is because I also have a water block. That tomshardware video makes the 6990 sound unusable with it stock cooling.

If I didn't have a water setup I would choose the gtx 590.

Both cards are showing the limitations of being stuck on 40nm which makes both cards a barrel of compromises.