Some quick comments.
- From a pure lens quality standpoint, I would say that the 3 Otus that I own are at least as good as the HC lenses. The gap is IMHO the largest comparing the HC 100mm f2.2 to the 55/85 mm. The 100mm HC is very good stopped down, but not in the same league wide open. The Otus 85mm f1.4 at f1.4 is probably better than the 100mm HC stopped down to f2.8. At f4 the Hasselblad lens becomes real good which makes sense since it is used most often at f5.6/f8 in studio applications.
- One problem with the 100mm HC is that specular highlights in bokeh area display hexagonal shapes that I don't find very pleasing. This is the result of the leaf shutter having only 6 blades that are not curved. Other than that its bokeh is very pleasing,
- The other HC lenses are very good, basically at the same level as the Otus, but of course with a lot less ambitious widest aperture. The 28mm, 50mm II, 300mm f4.5 are all brilliant, and are very sharp and well corrected for CA,
- Overall the image quality of the H6D-100c + HC is significantly superior to that of my D810 + Otus in terms of resolution, pixel quality (sharper on the Hasselblad images) - the combination of these 2 makes the real resolution gap probably closer to 130mp vs 36 - so almost 4 times more real world pixels. Besides, the colors, in particular in the greens, are really nice with the Hasselblad. Both natural and pleasing.
- In terms of the most important metric, dynamic range, both the D810 and the H6D-100c are brilliant. I would give a slight advantage to the Hasselblad since its shadows retain more color information in my book.
Since your interest is for an older Hasselblad back, the gap is likely to be much smaller. I would personally avoid the 5Ds due to its old generation low DR sensor, and go instead with a soon to be announced D820 if the rumors are confirmed. Unless you already own Canon mount Otis lenses of course.
Cheers,
Bernard