Well I personally have a Galaxy Nexus. But for a HTC One X vs Samsung Galaxy S3. I would choose the Galaxy S3 because:
1. Touchwiz is a lot more refined than Sense 4.0, so the user interface is less buggy and more intuitive.
2. S3 has a 2100mAh battery compared to the the 1800mAh in the HTC.
3. Better camera, both in lens quality and colour reproduction.
4. Removable battery and SD card.
5. Samsung Touchwiz is smoother than HTC Sense. So the phone will feel faster.
There are two things that the HTC One X does do better than the S3
1. The screen is LCD2, and imho looks better than the Super AMOLED screen on the S3, the colours are more vibrant and bright.
2. The plastic on the HTC feels higher quality than the one on the S3, but if you get a case that won't make the slightest difference.
But it's best to try out both phones at the store if you can.