It depends on the quad core. Mainly you see, the dual core A15 on the SIII is actually as powerful as the quad core A9s you see on the OneX and the international SIII because these are newer generation processors (which makes the Optimus G's quad A15s that much more powerful).
However, multiple core processors would really only matter depending on what you do with your device. My single core Galaxy S is still running happily even after two years, and mainly the only things it can't run well anymore are the newer games. So in your place, mainly due to LG's own history on unreliability and late updates (its dual core Optimus 2X actually got the Gingerbread update one year after the Galaxy S did), I'd pick the SIII or the OneX over it.