I think it makes a lot less difference then people realise. Unless the multiple rails are "made" seperately, multiple rail PSU start as a single large rail and split into smaller rails. "True" multi rail PSU exist, but are very expensive. In the end, it doesn't really matter.
Single rail is better and are generaly high quality unit. That said they are also more exspencive. The great majority of PSU availible have multiple current limited +12v rails. They are also cheaper and besause of that are the more widly used PSUs. Right now the top PSUs availible are single rail units from PC Power & Cooling.