For gaming, the 8gb kit would be slightly faster, but only barely. I would go for the 16gb kit as you might only get about 1FPS more with the 1866, but the 16gb kit would give more for the OS to use.
Looks like a good system too!
Would love to have it when you get that 6xx GPU and the IB.
EDIT: Corsair, Patriot, G. SKill, Kingston, Crucial, PNY, and Mushkin are all great RAM makers!