single rails deliver higher amps to single devices but if you hook many items to it you may find you need more connectors.
to put it simple, a psu is a transformer, it takes 120v power in and supply 12v amps to your system
a single rail means all leads come directly from the power supply output and share one connection.
multi rails are split on seperate connections for each rail and each rail is running through a limiting regulator so for safety reasons it cant carry too many amps (usually somewhere from 14-20 amps) the single rail is likewise protected but because it feeds everything this is set rather high and can cause melted wires, smoke, and even possably a small flame in the event of a direct short.
multi rail psu are safer for computers without devices needing over 20 amps and are safer in the event of direct shorting, which is rare but running wires in unintended places for hiding them can result in chaffed wires and cause direct shorts. they are safer because they trip faster in the event of overload or short.
single rail psu are better for higher amp devices that draw over 20 amps but because of this are just a tad slower in tripping in case of shorts. a single rail because it has a slightly higher trip threshold, does not protect your system as good as a multi rail would if the psu blew.