Hi all you 650b guys

Ok so I'm looking at buying a Giant Talon 27.5 4 hardtail so I can give this 650b thing a try. I've ridden 26er hardtails for about 5 months now. I'm 5'10" 160 lbs. I've done only a couple test rides at my LBS but don't know if I need a medium (18") or large (20") frame from this bike. I ride a spesh hardrock 19" frame right now, but I don't think I have enough experience to be able to tell which size frame is more comfortable. Do you guys have any insight or suggestions that could help me choose? Thanks!