I'd say get the FSP. It's more efficient, so it'll put out roughly the same wattage as the Seasonic. The only thing that I'm not so sure about is the amperage: the Aurum puts out 18A on two 12V rails, whereas the S12II puts out 40A on one 12V rail. I don't know how much of a difference that'd make, but truthfully you can't go wrong with either of these PSUs.
I would vote for the Seasonic. It is a known high quality unit.
It will deliver 40 amps vs. 36 for the fsp.
Nomally, you can't just add the amps from multiple rails, but in this case, you can.
What you do is look at the data plate which will give you the combined amps, or watts which you can divide by 12.
FSP is a good unit though, you would do well with either.
Dual rails is a safety feature which limits the amps that can be delivered. It really makes no diffrence at all.