The 590 still has a total of 3gb of VRAM and the additional cores make it far faster than a single 580 core clocked higher. And you can OC 590s too.
If it were a question about 2 x GTX 580 1.5GB vs GTX 590 2x1.5GB. I'd definately go with the GTX 590 as I would think it can comunicate better being so close? I am guessing, but I would like to think it's better than sharing across a ribbon.
If it were a question about 2 x GTX 580 3GB I might wait for a possible GTX 590 2x3GB. Though, I do not wish to spend that much money and will wait for some drop in prices.
Personally I want to buy them without heat exchangers for cheaper or with waterblocks for about the same price seeing as, I don't know I am guessing, vapour chambers are more expensive? Not pay more than the card with a vapour chamber plus a water block for one with a waterblock instead of a vapour chamber.