Single rail is nicer, since you don’t have to worry about anything, but as long as you’re careful about what goes on what rail it doesn’t matter much either way. Personally I perfer dual rail if I have alot of HDDs, as the spin up (even with staggered spin up) can cause a large drop on that rail, so having it independent is just a good idea to prevent voltage drops to more sensetive parts of the system such as the processors. Once under load, however, single rails tend to be more stable unless you have hybrid HDDs or other such start-stop loads going onto that rail single rail. The general idea is to make sure that the power going to your VRM and video cards is as ripple free as possible. As a side note: by alot of HDDs I mean 6-8 or more.
As for quad, that’s just a nice way to cut costs and still get a gazilion watts out of the system. Otherwise I see little point.