I wouldn't pick either if you're going to be locking yourself in for 2yrs.
The One X+ is a better phone, however, if you plan on putting an AOSP ROM on there, don't hold your breath.
I believe the ATT S3 has had the bootloader unlocked (someone correct me if I'm wrong, I haven't been following development on ATT phones) so you would have the option of potentially flashing an AOSP based ROM.
If I were in your position, I would wait and see what Q1 has to bring (or see my last statement). Should be lots of nice phones in the pipeline. The Tegra 3 is about a year old now, the S4 dual core is a beast, but it's quad core cousin, along with a much improved GPU is showing up in a few devices already.
If you're purely
looking at specs, neither the S3 or One X+ has the latest and greatest. It sounds like that's all you're comparing at this point and that's a terrible way to choose a phone. Go play with a few and see which one you like best.
Take a look at the LG Optimus G while you're there. Spec wise it blows both of those phones out of the water.
Forgot you're on ATT, take a look at the Nexus 4. No contract and you'll be hard pressed to find a review that doesn't praise it.