It might be more efficient, but it is not 80Plus Silver certified. http://www.anandtech.com/show/3985/three-550w-psus-for-different-prices/9
Shows it getting close to 80Plus Bronze levels.
As far as I'm concerned the HCG-520 is the better choice, but not because it is single rail. It is better because it is 80Plus Bronze rated, it is built by Seasonic - based on the positively reviewed Seasonic S12II Bronze, unfortunately I haven't seen the same calibre of review applied to the BP550. And finally it has longer (ATX12V, EPS12V, PCIe) cables.