Personally I like single rail designs, especially when you are looking at lower wattage supplies, because if you get in to a situation where you are using the power in the upper limits of the device then generally you have to be sure half of your components are on one rail vs the other but sometimes you don't know which connecters are attached to which rails (Generally the PCI-E will be on one, where other things are on the other or some logical separation).
Also, in some configurations in multirail designs you end up that your machine is fairly efficient but you want to jam a big video card in there... I ran in to this with a 6870 a while back using an Antec Earthwatts 380. The machine as tested peaked at about 120 watts the...