I went into the store on Friday to check out the GS3 and the HTC One X and ended up buying the One X over the GS3. While both phones are GREAT, and you can't really go wrong with either, I personally felt the One X was superior. While the GS3 has more features built into the phone (detect if your looking at screen to keep it on, watching a small video on top of whatever your doing, S-Voice, etc, etc), feels better in the hands and has the brand awareness of a Apple product with multiple companies producing multiple accessories for the device. The One X without no doubt has a much better build quality, and while the HTC screen is 0.1" smaller, the new LCD 2 screen is noticeably better when next to the pentile AMOLED screen on the GS3. Everything is just more clearer and sharper, the viewing angle is larger and the color representation is great.
Both HTC and Samsung have changed how Android 4.0 looks and acts on their respected devices, either improving or adding more features. HTC's Sense 4 is a huge improvement over Sense 3, removing a lot of bloat and making it much lighter from before and doesn't really take away the look or feel of ICS. While Samsung has improved Touchwiz, they seem to of missed the memo that GB is long gone and outdated. It is extremely difficult to know by looking at the phone that its actually running ICS due to the fact their Touchwiz interface looks exactly the same as the ones on their GB phones (GS2). Also the fact that they still have a dedicated menu button on the phone while ICS apps all have a built in on screen menu button points towards the GS3 being developed for GB and not ICS.
Performance wise they are about the same as they both run on the same CPU in the states, tho the GS3 does have a extra 1GB of RAM for a total of 2GB's!. HTC also spent A LOT of time incorporating a great camera into their phone. The dedicated image sense chip produces stunning life like photos each time with zero shutter lag. While the GS3 also has zero shutter lag, the multiple photo and video comparisons around the web all have the One X coming out on top of both the GS3 and even sometimes the iPhone 4S. Now the biggest downside to the One X is the lack of a expandable memory slot, sadly right now only the 16GB version of both the One X and the GS3 are available in the states while everyone else around the world has the bigger variants to choose from. This is one area where the GS3 trumps the One X due to its micro SD card slot. The GS3 has the ability to hold quadrupedal the amount of photos, video, apps, etc then the One X could ever dream of doing. HTC has partnered up with dropbox to provide a extra free 25GB of cloud storage to everyone who owns a One device. But this only means you'll have to deal with the insanely expensive and/or limited data caps/throttling on the carriers to have access to your data over their networks if you don't use WiFi. This alone could cause A LOT of people to turn away from the One X and choose the GS3 instead.
Again both are great devices and you can't go wrong with either. They both have their pros and cons, it's just up to the user to decide what he/she can live with.