Well there's a couple of things that come to mind when looking at each phone.
Hardware wise they are pretty similar, I think the ceramic casing of the HTC One-X is very cool though! I've heard some horror stories online though of the ceramic(the white one) retaining the color of whatever jeans you're wearing. Overall they both feel really good in your hands and power wise they are pretty similar.
I think what needs to looked at the hardest is the software side of the phone. HTC has Sense 4 which in my opinion is a step in the right direction, but still feels bloated and full of crap you'll never need. The Galaxy SIII has a much better arsenal of features, some noteworthy ones being S Voice which is completed integrated in the phone and all the features, very cool. Touchwiz isn't much better than sense though, I prefer stock over both, but that's just my opinion. Either way man there are tons of good articles online to help you decide, they are both excellent phones and neither of them is a wrong choice!
Enjoy your new phone!