My understanding of 8 phase versus 16 phase power is that the 16 phase will provide more stable voltage. Though, that may be true, from what I can gather 8 phase is more then enough to keep stable and OC.
It wasn't long ago that we were using 3 and even 2 phase power. Either way you don't see any reviews on it or even that much info from google searches, that tells me that it isn't a large factor in deciding between boards.