Its hard for me to imagine a situation where I would choose to use a shaft collar to transmit torque. There are keyless connectors designed for that purpose, and they generate a LOT more gripping force than a standard shaft collar.
As for the question of single bolt vs two bolt, I prefer the single bolt style if it will fit in the arrangement. They're simpler, generally more precise, and my gut tells me they should hold better, probably because of the closer initial fit and more surface contact.