I've always preferred single rail
PSU's, mainly because I don't know of many "true" multiple rail mainstream
PSU's. My other reasons would be that I run my systems heavy on the
CPU side of things but tend to stay away from multiple GPU setups, and I have stuck with the two
PSU makers that have never let me down, PCP&C and Zippy, both companies utilising a single high amperage rail..
From what I have seen in "most" multiple rail
PSU's it allows them run to lesser quality component as it won't be seeing the amperage that a single rail will..
For me it is more a stick with what I know works. I have never had a
PSU failure using Zippy or PCP&C