Okay, I flopped around before I made my choice. I ate $60 on sprint just to find I liked the S3 a lot more. Overall? The whole thing is a LOT smoother. The One X line literally struggled to be competitive with my Evo 3D, which I was 'upgrading' from.
This is all 'pre-patch' that was supposed to address the multi tasking issue that was present, but it was god awful. Rooting helped somewhat, but it was certainly a downgrade in overall speed from the 3D. I mean, the GPU was stronger, nicer camera, more metal construction, but there's a few things I hated.
One X:
The speaker is horrible compared to my 3D (which also sucked horribly)
The Multi-Tasking issues made the phone un-load things like the launcher, keyboard, and anything you need to use within 45 seconds (timed) of leaving an app.
The battery doesn't last as long as the S3.
S3:
The screen (opinion) isn't up to par with the One X. I'd much rather the One X LCD for overall use. Outside, the S3 is rather difficult to see in direct sunlight (issue for me).
The camera isn't near as nice as the One X. All being said though, it's better than everything else I've seen on the market, just not as nice as the One X.
The phone is totally plastic, and as a result dents very easily. 2 slow slides off my bed (not 2 feet off bubour carpet) have left 2 deep dents in the bezel.
Overall, I'd say take the S3 any day. However, the software fixes might have made the multi tasking problem much less prominent and more smimlar to the experience I had on the 3D (above average).
For a better feeling, most likely more durable phone, I would have to give it to the One X hands down. Much more of a premium built device. Stability and speed are on the S3 though. I didn't think the extra gigabyte of RAM would have as much an impact as it does. Sure, I'm never much over 1.17gb used, but that extra let's me play games in my N64 emulator that I could only dream of on any other phone (majoras mask, as a big one).
If anyone wants a clearer comparison, just post. I'm in my car right now, typing away on my phone. I'll be back in the barracks here shortly.