Fight Fight... Wait, I guess I shouldn't encourage it right
1) To me it is clear for 1200p and 1600p the 6990 wins by only a little -- so if you care about noise (I don't ) then get the 590 because you aren't losing much for a quieter rig.
2) At surround/eyefinity type resolutions the 6990 is the clear winner
3) Be careful when overclocking. On mature drivers there are plenty of 570s that have blown on 4+1 phases when people got aggressive. The issue is Fermi is power hungry... the drivers have little to do with it. Name ONE card where reviewers reported blowing up the card while testing OCs in recent history. I certainly haven't see it.
Overall I give the 6990 a slight edge because this is OCN and we care about performance not noise, right...