I love them both. Since my SG not a Standard, I'm veering slightly from the original question - these two guitars are not really apples and apples. With p-90s, I'd have to say the SG is more versatile than either a Std or an S-100. The blues/rock credentials of the S-100 are undeniable. If I had to keep just one, it would definitely be the S-100 - I have a Strat to cover other bases when I want to and in fact the SG is for sale.
I find the SG is a bit easier to tune correctly - that may just be that the S-100 intonation needs a bit of tweaking. The S-100 has a looser feel when bending notes - all that extra string length behind the bridge. Note the little piece of grey foam to dampen ghost notes.
Acoustically, the S-100 is brighter - keeping in mind that this SG model has a fat neck which may darken the tone a bit. The S-100 is a slightly lighter and better balanced than the SG which is definitely a little neck heavy. That said - with a suede strap I've never had the SG "neck-dive" on me.
For overall coolness, mojo, individuality, character (or whatever you want to call it) the S-100 wins hands-down!