Single rail is more forgiving, it allows installation without planning as far as power budget goes. If the supply can make enough power, and the wires are thick enough to deliver it without loss, you are good.
Multi rail demands more thought and engineering knowledge. You have to make sure that each rail is matched to the loads that you need it to run, and many times the stock configuration of a multi rail power supply doesn't allow enough amps to reach the SATA power connectors. That means either sourcing different cables for a modular supply, or building them yourself if the manufacturer doesn't provide that configuration, or cutting and splicing a non-modular power supply.
Reconfiguring a multi rail supply is WAY above many people's pay grade, so the standard answer for people wanting to run large numbers of hard drives on consumer grade power supplies is to go single rail.