hey _33 multi-rail units are not generally better power supplys. it all depends on the design. some multi-rail designs completely separate the various rails so if one is maxed out and the other has no draw you are wasting half your power. other designs have capped draw per rail but will still share current right up to the cap.
single rail designs are still the general favorite design by overclockers as no matter what you can use every watt available. either way posting a question about a certain psu will give you a good about of feedback from that unit.