The 6990 is slightly ahead, but they're basically equal.
The 6990 uses less power, the 590 makes less noise.
I think the 6990 has more overclocking headroom though - there have been several 590's dying because the VRM's couldn't handle the levels of power used.
Nvidia says their newer drivers will limit current to prevent that from happening (And I'd expect would also limit performance).
I think the GF114 would be better suited to a multi-GPU card than GF110.
As for me, I'd prefer to get a 28nm GPU.
TSMC haven't exactly executed things that well in the past year or two though, which is why Nvidia and AMD are stuck on 40nm, with the higher power consumption and cost to manufacture the GPU's going with it.
Hopefully AMD starts using GF instead of TSMC. Nvidia probably won't (though I'd like to see that too. It would benefit both AMD and Nvidia).