"What's the difference between LSX and SBC?"
In one word...EVERYTHING.
In more words, everything except a couple of measurements, most of the bellhousing bolt pattern, and I think the early LS shared rod bearings with the SBC.
It's about one step away from asking "What's the difference between the Chevy 350 and the Ford 351?", or "What's the difference between the Chrysler 360 and the AMC 360?"
GM pushes the (incorrect and deceptive) notion that the LS is just another development of the (Chevy) small-block. When it was new, GM was calling it a "Mid-block" because it was bigger than the SBC and smaller than the BBC. And the LS isn't a "Chevy" engine at all--it was developed from the beginning as a "Corporate" engine by GM Powertrain.