Jonnyguru rates the NZXT better. In terms of raw numbers (benchmarks) it is indeed better.
However I am not sure as to the reliability of NZXT. As an example, the Vertex2 benchmarks higher than the other SSDs in it's price range. Yet it fails 30% of the time, lol.
your doing your calculations wrong. The PSU will always supply the rated w if its a good psu, but it will draw the 950w from the wall outlet. So if your 800w max calc is for both cards and system running at max then I would just get an 850w psu
The NZXT is build by Superflower, more recent JG reviews have build quality scores and I'm pretty sure Superflower built units have been getting high build quality scores. So I don't think there will be an issue with reliability with either unit.
So the NZXT would probably come out ahead due to it's better performance numbers.
~1000W is a good choice for a system with two GTX 580s IMO.
I got the NZXT unit and it is Really nice. You will like that unit and the white paint job really does make it stand out compared to the usual black that most units look now days. It is slightly longer than most typical units but should be of any concern unless crappy case.