From what I understand, the SOCOM is louder, and if you're looking for the one of the two that is best capable of hitting targets at a distance, the full size is for you. However, you already know this stuff. Why are you asking? Personally, if there had been a Scout/Squad M1A available when I went looking for mine, I probably would have bought that one. 18" barrel, but they didn't have to redesign the gas system like they did for the SOCOM.
People Eater, you own a SOCOM (or did, depending if you sold it or not). Can you think of any other major differences?
Mountain, why are you buying it, again?