The SGS3 is coming with two processors - Exynos 4412 (quad you know) and, yep, the Qualcomm S4, just like the ATT One X.
In order to signal, it takes the cpu, modem and radio transceivers.
The S4 has a world modem built in. It's programmable to support GSM, EDGE, GPRS, CDMA, LTE, HSPA+ and is part of the same block that also supports BT, WIFI, GPS, GLONASS.
Parts count down, costs down, reliability up.
The only people terribly concerned are those who care bunches about synthetic benchmarks - and if you find me one that says one processor wins something, guaranteed I can find another with opposite results.
And depending on which version (the SGS2 wasn't a model, it was a series of them, same will be true for the SGS3), it's going to come down to comparing the One X with the S4 vs the SGS3 with the S4.
~~~~~~~
If you are comparing the SGS2, not 3, to dual core One X, it's not just about core numbers. The S4 is using the new generation of cpu cores, all the others are using the existing A9 type cpu. Depending on which SGS2 in the USA, it may contain the Qualcomm S3 (it's all about the modems), in which case, game over - the One X wins hands down.