Do you already have a 308AR ? If not , I would go for the 308 , just because of ammo availability . Those other calibers sound great , until you want ammo for them . You can get 308 anywhere , wally world , K-mart , or any sporting goods store out in the boonies , if your hunting.
I like the ballistics of 6.8 , & 6.5 CM , just don't know much about them . If the 6.5 CM is your choice , get into reloading & most of the ammo problems go away.