Which of these two for is better for Sli?

alquemist

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Oct 11, 2011
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I've been doing allot of research and looking at different mother boards. I came across liking these two. The ASUS P8Z77-V LK LGA and ASRock Z77 Extreme3 LGA. Now here is the thing that has me more confused is:

Asus has PCI Express 3.0: x16 2 (x16/0 or x8/x8)
and
ASRock has PCI Express 3.0: x16: 2 (x16/x8 or x8/x8)

Im guessing the will perform better with 2 GPU in sli at x16/x8?? Or am mistaken?
I will be running 2x GTX 680 btw.