I'm getting tired of waiting for the 550W Gold model. Just noticed that the S12D-750 is $115 at newegg
and the 850W version is $120.
That makes it cheap enough to get the Silver 750W instead of the Gold 650W and as long as you aren't breaking 200W ever under any circumstances (which is true for all the PCs in my house) I don't expect to be able to hear the difference.
The savings of $50 up front will outweigh the extra half a percent efficiency at the loads I'll use making the Silver more cost effective over the life of the unit.
The only down side is that it has 4 PCIe connectors and I'll only need 1 at most for any video card I'd buy. So I'll have some cable clutter. But to save $50 I'll tie up some cables and shove them out of the way.