The 590 is marginally better than the 6990. If anyone tells you otherwise, it's because they're remembering back to the days when the 590 was first released and everyone reviewed it on terrible drivers. As a result, it looked like the 6990 stomped it in everything. If you looked at a 590 review that's been done in the last six months (essentially anytime after the first week of its release) you'd see that it trades blows with the 6990 and edges it out in a few tests.
EDIT: Also the 590 is quieter than the 6990, not quiet in comparison to a 560 or 6850 though.