I've posted this elsewhere but I think it is most relevant here at AndroidForums. This information is from various sources, but mainly from GSM. So don't flame me if it doesn't fit your opinions because I'm not being biased in any sort of way.
Samsung Galaxy S3
Pros
Cons
HTC One X
Pros
Cons
Similarities between the two phones
Features depending on user-preference
The Samsung Galaxy S3 beats the HTC One X in the CPU and GPU benchmarks. The new Exynos 4 processor of the Samsung Galaxy S3 is stronger than the Tegra 3 of the HTC One X.
There are 2 versions for each smartphone. Both come in an International version and a 4G/LTE version.
Samsung Galaxy S3
International Version
CPU: Exynos 4 quad core processor
RAM: 1GB RAM
GPU: ARM Mali-400
US or 4G/LTE Version
CPU: Snapdragon S4 "Krait"
RAM: 2GB RAM
GPU: Qualcomm Adreno 225
HTC One X
International Version
CPU: Tegra 3 quad core processor
RAM: 1GB RAM
GPU: ULP Geforce
US or 4G/LTE Version
CPU: Snapdragon S4 "Krait"
RAM: 1GB RAM
GPU: Qualcomm Adreno 225
Which CPU and GPU is better and why?
In terms of processing power, the Exynos 4th generation quad core processor of the Samsung Galaxy S3 is much better than the Tegra 3 quad core processor of the HTC One X. The ARM Mali-400 GPU on the Samsung Galaxy S3 is also superior compared to the ULP Geforce of the HTC One X.
However if we're comparing the US or 4G/LTE versions, then this gap closes significantly. Some US providers will have 2GB RAM versions of the Samsung Galaxy S3 4G/LTE so the SGS3 may win in this case. However it's still an extremely close battle and any difference will most likely be from the levels of optimization of the software engines.
Browser speed test (BrowserMark)

Source: Samsung Galaxy S3 (GT-i9300) BrowserMark HTML5 Benchmark Test - GSM Insider : Gadgets news from the insider
What this basically says is that the Samsung Galaxy S3 has the fastest web browsing speeds on the market. This could be due to the SGS3's superior software engine and better signal transmission hardware. The HTC One X is not far behind though and is much better than the HTC Sensation and Sony Xperia S.
3G/4G LTE Signal transmission tests:
BrowserMark - web browser speed tests

CPU/GPU tests:
GLBenchmark 2.1 (Egypt)


Quadrant

Sunspider

Camera Quality
Source: GSM
Video Quality
Source: GSM
Battery Life
The Samsung Galaxy S3 has the superior battery life out of the two. Refer to these links from GSM:


The Samsung Galaxy S3 has the better battery life and the difference is stunning for video playback.
Other tests and comparisons
Bootup speed test
http://www.youtube.com/watch?v=OkPrBrGQlW4
Video capture test
http://www.youtube.com/watch?v=WAJZZlQ8xkw
19FPS lag on the HTC One X
http://www.youtube.com/watch?v=c3wCQbcp4Qk
Design and user interfaces comparisons
http://www.youtube.com/watch?v=6MFCx4z6LCQ
Samsung Galaxy S3 v HTC One X
Samsung Galaxy S3
Pros
- Super AMOLED HD Screen is excellent (superior blacks, higher contrast ratios and viewing angles)
- Higher display contrast and better sunlight legibility
- More storage options - Samsung Galaxy S3 offers 16/32/64GB + 64GB microSD which brings it to a maximum of 128GB compared to the HTC One X's non-expandable 32GB
- Higher quality camera sensor and lens compared to HTC One X
- Fastest web browser speed in the market. Evidence: http://1.bp.blogspot.com/-00VpL_oIWJs/T6NalRAQnzI/AAAAAAAAHzQ/CVLZAXdu0gI/s1600/t3.jpg
- 4G/LTE version has 2GB RAM which is better than the 1GB RAM of the 4G/LTE HTC One X
- Comes with a cool and useful barometer
- Faster and better GPU
- Louder and better audio player compared to HTC One X
- Updated to Android 4.1 Jellybean faster (August/September 2012)
- S Voice natural language commands and dictation
- S Beam for easy and fast file transfers
- User-accessible 2100mAh battery over non-removable 1800mAh battery
- 1.9MP front-facing camera with Smart Stay
Cons
- Design is seen as too conservative by some
- More expensive than HTC One X
- Whites have a very slight blue tint to them
HTC One X
Pros
- Beats audio
- Already on the market
- Cheaper
- Design looks great
- Very good screen (LCD 2) which is excellent (higher max brightness and more accurate whites)
- Quad core with 1GB RAM is faster than most phones on the market
- f2.0 aperture means the HTC One X's camera is better in dark conditions
- Sense UI looks great
Cons
- Camera quality is quite low compared to the SGS3
- Only comes in 32GB storage version and has no microSD... so the maximum storage is 32GB
- 4G/LTE version does not come with 2GB RAM
- Sense is not optimized so the UI is significantly laggier
- Video capture is not at 30FPS, and there is evidence that it is 20FPS which is very bad
- Blacks aren't as strong as on the SGS3 but this is almost a non-issue
Similarities between the two phones
- The build quality of both phones are about the same as they both use polycarbonate (there is evidence from The Verge, GSM and Engadget). The only difference is that the Samsung Galaxy S3 has a metallic trim on the sides. The white version of the SGS3 has a glossy marble-like finish, whereas the pebble blue version has a metallic finish. The HTC One X's battery is not replaceable so you can't replace it with a more powerful battery.
- Both phones are actually the fastest phones on the market
- Both phones are superior to other competitors in terms of browser speeds.
Features depending on user-preference
- 4.8" screen of the Samsung Galaxy S3 vs the 4.7" screen of the HTC One X
- Sense vs TouchWiz. Sense looks great, whereas TouchWiz is a bit boring. However TouchWiz is more responsive and quicker
- Super AMOLED HD vs SLCD 2. The Samsung Galaxy S3's Super AMOLED HD has higher contrast ratios, better blacks and viewing angles. Super AMOLED HD also has better outdoor visibility according to GSM. The HTC One X's SLCD 2 screen, however, has higher max brightness and more accurate whites.
The Samsung Galaxy S3 beats the HTC One X in the CPU and GPU benchmarks. The new Exynos 4 processor of the Samsung Galaxy S3 is stronger than the Tegra 3 of the HTC One X.
There are 2 versions for each smartphone. Both come in an International version and a 4G/LTE version.
Samsung Galaxy S3
International Version
CPU: Exynos 4 quad core processor
RAM: 1GB RAM
GPU: ARM Mali-400
US or 4G/LTE Version
CPU: Snapdragon S4 "Krait"
RAM: 2GB RAM
GPU: Qualcomm Adreno 225
HTC One X
International Version
CPU: Tegra 3 quad core processor
RAM: 1GB RAM
GPU: ULP Geforce
US or 4G/LTE Version
CPU: Snapdragon S4 "Krait"
RAM: 1GB RAM
GPU: Qualcomm Adreno 225
Which CPU and GPU is better and why?
In terms of processing power, the Exynos 4th generation quad core processor of the Samsung Galaxy S3 is much better than the Tegra 3 quad core processor of the HTC One X. The ARM Mali-400 GPU on the Samsung Galaxy S3 is also superior compared to the ULP Geforce of the HTC One X.
However if we're comparing the US or 4G/LTE versions, then this gap closes significantly. Some US providers will have 2GB RAM versions of the Samsung Galaxy S3 4G/LTE so the SGS3 may win in this case. However it's still an extremely close battle and any difference will most likely be from the levels of optimization of the software engines.
Browser speed test (BrowserMark)
Source: Samsung Galaxy S3 (GT-i9300) BrowserMark HTML5 Benchmark Test - GSM Insider : Gadgets news from the insider
What this basically says is that the Samsung Galaxy S3 has the fastest web browsing speeds on the market. This could be due to the SGS3's superior software engine and better signal transmission hardware. The HTC One X is not far behind though and is much better than the HTC Sensation and Sony Xperia S.
3G/4G LTE Signal transmission tests:
BrowserMark - web browser speed tests
CPU/GPU tests:
GLBenchmark 2.1 (Egypt)
Quadrant
Sunspider
Camera Quality
Source: GSM
Image quality
Both the Samsung Galaxy S III and the HTC One X feature 8 MP cameras that are capable of taking photos at max resolution of 3264x2448 pixels. Both feature single LED on the back and, thanks to their zippy processors, both offer 1080p and 720p video recording with simultaneous image capture. The lens used in the Galaxy S III has a slightly wider filed of view compared to the One X.
So let's dive right into the image quality. You'll notice that the weather wasn't identical for the two contenders, but lighting conditions were similar enough to make the images comparable.
We added the Galaxy S II to the first round of tests for good measure. As you can see from the crops, in good lighting the Galaxy S III tends to produce images with more detail than both its predecessor and the HTC One X.
The processing of the new Samsung flagship is much more mature too - it introduces very few oversharpening artifacts and loses little detail, while still managing to keep the noise under control. The One X on the other hand is a bit too aggressive with the sharpening and isn't doing nearly as good at removing the noise.
Both contenders tend to oversaturate colors (particularly green), but the Galaxy S III comes a little closer to reality.
And there go a couple of more shots, which tell a similar story. Differences aren't huge, but the Galaxy S III still has an edge.
We also tested the photos that the One X and the Galaxy S III captured, while recording video. They are 6 megapixel stills in 16:9 aspect ratio.
It's interesting to note that the HTC One X frames pictures differently from the video so you always end up with a result with a wider field of view. The Galaxy S III in-video shots have the same framing as the video.
In good light, the two smartphones resolve about the same amount of detail here, but the One X has better contrast and it sharpens its images more aggressively, which makes its output look punchier. The Galaxy S III still has slightly better color reproduction, though.
Things get vastly different in low light, when the bright F/2.0 optics of One X count big time. In such conditions the HTC flagship crushes its opponent, producing images with far less noise and much better contrast.
Most devices out there offer HDR mode straight out of the camera app and so do the Galaxy S III and HTC One X. The One X produces more dramatic looking results, while the Galaxy S III has a pretty subtle approach. Which solution is better is strictly a matter of personal preference, but the Galaxy S III does resolve more detail and its highlights aren't as exaggerated.
Photo quality comparison
We have the Samsung Galaxy S III and HTC One X in our Photo comparison tool. The tool's page will give you the full details and it's where you can make the most comprehensive observations on the cameras involved in this shootout.
The Galaxy S III uses its resolution advantage to easily outdo the HTC One X in the first two charts. The two are much more evenly matched on the last studio challenge, but the S III shot has far less noise there.
Video Quality
Source: GSM
We were pleased with the video capture on the HTC One X. But the Galaxy S III is just too good to compete with - the resolved detail in the Galaxy S III videos is incredible and they are very smooth with accurate colors.
Here's the Galaxy S III 1080p FullHD sample.
http://www.youtube.com/watch?v=Aif8pLLjDJs
And the One X 1080p video sample.
http://www.youtube.com/watch?v=LfBRkR682rg
There's also the bitrate to take into account. The HTC One X videos come out with a relatively low bitrate of about 10 Mbps, while the Galaxy S III's clips hover around the 17 Mbps mark. Both devices offer stereo sound recording during capture. Below we've lined up 720p video samples by each of the devices.
http://www.youtube.com/watch?v=xr-BdysccZM
The lower bitrate is evident here as well. At 6Mbps the One X videos have about half the bitrate of those captured with the Galaxy S III. (Galaxy S3 is the winner)
http://www.youtube.com/watch?v=Ee0qO4Z9u2c
Overall, it's hard to see beyond the Samsung I9300 Galaxy S III for the winner in the video recording category. The One X shoots decent videos, but those are nowhere near the S III standard.
Battery Life
The Samsung Galaxy S3 has the superior battery life out of the two. Refer to these links from GSM:
Source: GSMThe HTC One X suffered a humiliation in the video playback part of our test. The Galaxy S III put the superior power efficiency of its screen to great use here and took a minute over 10 hours to deplete 90% of its battery. The HTC One X, on the other hand, gave up after less than 6 hours of SD playback.
The Samsung Galaxy S3 has the better battery life and the difference is stunning for video playback.
Other tests and comparisons
Bootup speed test
http://www.youtube.com/watch?v=OkPrBrGQlW4
Video capture test
http://www.youtube.com/watch?v=WAJZZlQ8xkw
19FPS lag on the HTC One X
http://www.youtube.com/watch?v=c3wCQbcp4Qk
Design and user interfaces comparisons
http://www.youtube.com/watch?v=6MFCx4z6LCQ