Noone officially has 4.2 on their s3 bud. Theres a leaked samsung build for the international s3 and theres loads of perfect aosp based 4.2 custom roms. Even the 4.3 ones are getting really stable.
Seems like samsung will probably go straight to 4.3 but depending what carrier youre on, you could be waiting till next year.
If you can live without the samsung features, flashing a nice 4.2 rom is definately the way to go to make your s3 perform to its full potential. Itl feel FAR faster than a stock s4 for example :thumbup: