it is the neverending question I am asking myself...what is the difference, how big is it etc.
After using M9 and M240 side by side, and S006 and S007 as well, direct comperisons could not often prove my general feeling (that CCD gives better color in general).
Also in direct comparison, for example between SL2+SL75 vs Leica S vs x1d it is not so clear in each image, that medium format is ahead.
On the other side, if I use the x1d or Leica S I feel overall that I am very fine with the IQ I get, very 3D, very smooth, specially in the midtones, very "un-digital". Detail without looking oversharpened etc. The Leica S IQ is even a little more rewarding for my taste, specially the sharpness rolloff. I am not totally happy with the AF of the S though. The x1dII AF works much more reliable for my use, and I can move focus point around.
Overall I feel even the small medium format makes it easier for me to get color, specially also skincolor, midtones and a look, which comes very close to what I see with my eyes in reality.
On th other side I am sure that one could show me images from SL2+50/1.4 SL vs S or x1d images where I might not detect, which one is from which camera.