For those who are not fuji fans, the x-trans II and III are the sensors in the fuji interchangeable lens series.They are both aps-c size sensors mesuring 23.6 x 15.6 mm (compared to what is called full frame that measure 36×24 cm). The first 16 megapixel x-trans sensor came out in 2012 on the x-pro 1 camera and was also used in the x-e1 and x-m1. In january 2013, the x100s, successor of the orginial x100, was introduced, sporting the new x-Trans II sensor. In October 2013, the x-e2 was introduced using the same sensor, then the x-t1 (2014), x100T (2014) and x-t10 (2015)..
As of January 2016, the X-Trans III (24 megapixel) is the new shiny sensor in the Fuji world. This sensor is also used in the new X-T2 (September 2016).
I have recently upgraded from the X100T (the one with the hybrid viewfinder) to a X-Pro 2 (and the new hybrid viewfinder). The main reason I upgraded is the newly announced 23mm f/2 lens. It is calling to me already! I can now have a x100T-like camera with weather sealing and the abilty to carry an extra lens if I feel the need. And sometimes I feel like using the 35mm feld of view for my street photography, sometimes the 50mm.
The body of the x-pro 2 is heavier however. (440g for the x100T lens and all vs 495g body only). We are talking full magnesium alloy for the x-pro 2 body and and mixture of aluminium and magnesium alloy. Don’t get me wrong, the x100T is solid, but the x-pro2 feels good!
So now on to image quality : from 16mp to 24mp. With all the hype around the x-pro2 and x-t2, I’m entitled to thinking I’m going to get a whole new level of picture quality, so I took a few pictures with my x-pro 2 and with my x-t1 (because I sold the x100T but they use the same sensor). The lens I used was my beautiful 35mm f/1.4 and the 23mm f/1.4.
1) Details

I took a photo of the same closed down pub in our town, one with the x-pro 2 and one with the x-t1. I then did a 100% zoom on each picture.

The difference in detail is obvious. The photo on the x-pro 2 is 6000 x 4000 pixels. On the x-t1 it is 4896 x 3264. This is very good news for printing very large formats and for those who like to crop their images while editing. You can take off 1/3 of the picture and get the 16mp of the x-trans II sensor. I like macro photography too, and in that field framing is not always very easy to get right.
2) Dynamic range.
I overexposed by 3 stops the following photo.

In Lightroom, I then reduced the exposure by 2 stops and used the Hightlights slider at 0 to recover what I could. Here are the results:

Yep! Not much differnce there! The skies are white and the cobble stones too. What is gone is gone forever… so don’t overexpose your highlights. I’ve found that I can recover about 1 stop and thats it.
I also did a shadow recovery test but the x-t1 photos came out with a lot of flare…So here is a before and after shot on the x-pro 2 shooting into the sun.

I added 1.7 stops of exposure, set the shadows to 100 and the highlights to 0. No hdr needed here! The x-trans sensors have excellent dynamic range, the shadow recovery is extraordinary.
The next photo is taken in the doorway of a local museum.

With the same settings, the x-pro 2 shows more detail in the shadows which gives a natural look to the photo while retaining detail. When I pushed the shadows up and highlights down, here is what I got.

The same picture on both cameras! I can’t see much difference here. I’d say the dynamic range is the same in real world use.
3) Noise
I went into our Cathedral and shot a statue at ISO 6400.

This is a slightly cropped version with the x-t1 (16 mp x-trans II) sensor. I find it very usable indeed. Notice the blow out highlights wooden base (on the right). At ISO 200, I think I could have pulled those back but here at ISO 6400 they are gone.
Here are a couple of crops at 100%. I had to downsample the 24mp x-pto2 photo to show the same amount of detail.

I can’t see much difference… and when I try the x-pro 2 at ISO 12800 (the x-trans II sensor can go up to 6400 max in raw)

The noise is noticeable worse here but the detail is still there mostly and with a bit of processing I think I could get a decent quality picture out of this.
Conclusion
A quick look at the different aspects reveal little difference between both sensors in dynamic range and in noise levels. The huge difference is the pixel count. There is a 50% increase between the 16mp x-trans II sensor and the 24mp x-trans III sensor. It is no small feat to keep the same image quality while reducing pixel pitch. I imagine the new X Processor Pro is working hard in the background !
My conclusion is simple : if the only reason to upgrade to a camera with the new sensor is the image quality then cost seems unreasonable. I have got large prints (70cm x 50cm) with the x-t1 (and also with a micro four thirds olympus e-m5) and cannot see any failure in the image quality.
The major upgrade from the x100T to the x-pro 2 (apart from being able to change the lens) is the autofocus, the new hybrid viewfinder, the dual sd slots, the new layout of buttons (I can do everything with one hand), the new menus, the mult-meter exposure setting, the joystick to choose autofocus points, the number of configurable buttons, the new acros film simulation, the new mechanical shutter that goes to 1/8000s … the list goes on and on. The x-pro 2 is part of a new generation of cameras.