I did all mine under 1. Account bound items, such as heirlooms for leveling, can be traded to any of my accounts. Single authenticator for all accounts. There's probably some other stuff like transferring toons between accounts, but I've never done that, so I'm not sure.

I know that the new yearly contract thing they did has per-Battlenet restrictions, so that might be a reason to go with separate Battlenets, but I can't think of anything else offhand.