My wife has the One X on AT&T and I have the SGS3 on Verizon. The S3 definitely has some key plusses (microSD, removable battery, and RAM), but the One X is, IMO, a much higher quality phone. The screen is night and day in favor of the One X, and, since that's the way you interface with the phone, that outweighs the other features for me. Also, I much prefer the build and in-hand feel of the One X. The textured ceramic feels great, and the way the screen sits above the rest of the phone is swanky. The GS3 feels very pedestrian in comparison.
Now, if you're comparing international variants, you can't ignore the Exynos in the GS3, but, on the same silicon, I'd go for the One X. Too bad it's not on Verizon...