Since ‘surrounds’ are two speakers signals, not four, I’d go with the two eight inch. But then, if the majority of use was music, instead of surround/playback, I might go the other way. It really depends on usage, room, and your ears. 

I would also find a way to have the speakers point straight down since you mention "vaulted".

Thanks guys 

I have seen some diagrams using 7.1 system. Two speakers behind and two just in front of the seating area. Probably just go for the two 8’s behind. 

Sonos doesn’t currently support 7.1 directly, although they do support Atmos, using up to 5.2 speakers.