Broadly speaking: the main differences between an early/mid '70s S-100 and the vast majority of SGs ever are the neck/body angle (S-100s are very shallow while SGs are almost always steeper), the neck width at the nut (S-100s 1 10/16", SGs from 1 11/16" down to 1 9/16"), bridge construction (S-100s attached to unanchored posts sitting atop metal cups & with roller saddles, SGs attached to posts screwed into the body & with slotted TOM saddles) and pickups. S-100s tend to be spankier and even kinda plunky acoustically, not unlike Rickenbackers, while SGs are more compressed and sustain-y. Plugged in this translates to the S-100s having a sharper attack, again not unlike a Rickenbacker's percussive kerrang. SGs sustain louder into an amp, though not really longer.
IMO S-100s play & feel more like a blend of Tele & Rickie (which I love!) while SGs are typically Gibson. S-100s are my fav Guild solidbodies…same goes for SGs in the Gibson line.
-Dave-