$5, mainly. The Antec has four 12V rails @ 25A, while the Corsair lists one @ 60A.
It supports SLI and CrossfireX (three PCI-E slots), plenty of SATA ports. All current i7 boards use the x58 chipset.
$6. 8-8-8-24 timing vs 9-9--9-24 (lower is better). One looks cooler.
Used to mount the cooler, as the LGA 1366 is not native to most coolers on the market.
More is better. The Black series are very fast and great for OS drives. The Green is more power efficient and better for files.
A single card solution is always better than a multi-card one whenever possible. With the 5870 you could always add another one or two down the road, or mix and match another ATI card(that is possible now). The 5870 is the best out for now. The 295 is a touch faster, but more expensive and last gen.
Great minds thing alike.
I defer to those who know what they are talking about.