Note: The images in this review were shot in raw and processed to taste using Adobe Lightroomm 5 Beta and exported to JPEG. Changes have been made to exposure, highlights, shadows, clarity and vibrance, but no noise reduction has been applied.
With a completely different sensor that eschews the optical low-pass filter found in the X100, the X100S should - in theory - deliver images with more detail. When comparing images taken with both the X100S and the X100, you might be hard-pressed to find any meaningful differences in the amount of detail captured by either camera if you don't look up close. If you squint, you can tell that the X100S captures more detail without the blurriness that you'd usually see when looking very close at a photo captured with an optical low-pass filter.
In the two examples above, we see that the X100S' image does indeed capture more fine detail. But in the X100S' image, processed from a raw file within Lightroom 5, you can see how some of the crystals are rendered oddly. Take a look at the two images below, both 100% crops from raw files, with the X100S' 16MP image down-sampled to match the X100's 12MP. Can you tell which image was taken by which camera?
(The first image was taken by the X100, the second by the X100S.) You can see that the X100S captures more fine detail than the X100; the question to ask then is if the increase in clarity is worth the upgrade. If you already own a X100, the difference might be minute, and probably isn't a good enough reason by itself to buy a new X100S. Also, the raw files from the X100's conventional APS-C sensor plays better with raw converters like Lightroom right now, without any of the rendering artifacts that can happen with the X100S' raw files.
When comparing the two cameras' ISO performance, we found them to be on par with each other. Fujifilm seems to have some magic formula for its sensors, as the X100's conventional sensor and the X-Trans sensors have the best high ISO, low noise performance we've seen in any APS-C sensor. Grain is kept to a minimum, what's there has an organic quality to it and it's mostly luminance grain, with little if any color noise. With both cameras you could shoot up ISO 3200 with confidence, and even up to ISO 6400 with some noise reduction.
When shooting our night test scene, we observed something interesting: Moire showed up in a small section of the X100's images (in the fence around the playground), while none showed in the X100S'. We're hesitant to reach for a conclusion with such a small sample set, but it seems to lend weight to the rumors that the X100 ships with a weak optical low-pass filter, explaining its ability to capture so much fine detail. It also gives the X100S' X-Trans II sensor bonus points for its ability to avoid moire artifacts even without an optical low-pass filter.