So which is better type of power supply is better--single-rail or multi-rail? Neither, usually. From a performance standpoint, both work equally well; and in general both are very safe to use. If you're building an especially powerful system, though, multi-rail OCP provides an extra layer of safety in case something short-circuits, lessening the odds of frying your costly components during a computing catastrophe.
Cabling: Piecemeal or whole hog?
Another consideration is cabling. Power supplies are available with hard-wired cabling, with partially modular cabling, or with fully modular cabling. In modular power supplies, you can add or remove cabling from the PSU as needed to avoid case clutter.
Technically, a power supply with hard-wired cabling is optimal because it requires no additional connections between the unit's internal PCB and the connector that will ultimately be plugged into one of your components. One end of the cable is soldered into the PSU's PCB and the other end terminates in a standard connector, with no breaks in the line. Whenever you introduce an additional connection between the PSU and your components--as happens with modular power supplies--you add more resistance and another potential point of failure into the line; and any increase in resistance translates into lost efficiency.
That said, the additional resistance is normally minimal and not a cause for concern for most users. Meanwhile, modular cabling greatly simplifies keeping the interior of your case nice and clean--just don't connect any superfluous cables to keep the clutter down. Most people prefer modular PSUs, though they cost a bit more than nonmodular models.