All this is strictly personal preferences. It depends on what you may see, or not see in how each system renders light, details, etc., plus the functional aspects of operation, or whatever measure you personally use. It is hard to please a portrait subject or client, if you can't first please yourself.
I do what you do Bruno, mostly people photography, and a lot of portrait work with and without lighting. I've been a wedding and portrait photographer forever, but am now retiring from wedding work for the most part, and concentrating on portraits. A lot of wedding assignments also involve portrait work, especially Bridal portraits.
I simply do not like using a 35mm DSLR for this sort of photography if I can help it. I don't shoot portraits in bad light, so do not need very high ISO abilities. I don't like squinting through a tiny peep-hole finders, and much prefer big, lush focusing screens.
While I am sure cameras like the D800 or Canon 5D-III are terrific and have their devoted users producing great work, I simply do not like how Nikon or Canon glass renders, preferring the Hasselblad glass over that, and the Zeiss/Schneider lenses over that, and the Leica optics over all of them ... be they M, R or S. Plus, I like AF ability for more spontaneous portrait approaches ... so, adapting Leica R, or using Zeiss MF lenses on a Canon or Nikon is a no go for me ... been there, done that to death ... way to slow for moving subjects when doing spontaneous or environmental candid portraiture.
In general, I do like most any of the MFD systems for portrait work ... a lot. Big bright screens, better and better optics, ever improving ergonomics. Each one has their pluses and minuses, but as a whole the possibilities are wonderful.
My pal uses a Contax 645 and Phase back, another uses a H4D/50 to shoot all of his people work, I've used a Mamiya with a Leaf Aptus back and a bunch of Hasselbald H cameras all the way to a H4D/60 ... and now have centered on the S system which does everything I want and then some ... basically my dream kit come to life. It is fast as hell to operate, uses dual shutter for ambient work or with HSS lighting. It can take the S lenses, Hasselblad lenses, and now the Zeiss Contax 645 lenses ... all while retaining full functions including AF.
Today's Portrait photographer can span a hug range of approaches, however, one thing that tends to be a general commonality is use of post processing variations. It is here, in post, that more of the differences began revealing themselves IMHO. Personally, I've come to prefer optics that render using micro-contrast for acuity ... it seems to stand up to many of the Portrait type post programs available to the portrait photographer these days. I find it very apparent in the skin texture ... which is no small issue for this work.
In particular, the S-120/2.5 is a unique optic for portraits ... very fast for a 120 MF lens with gorgeous OOF rendering and that micro contrast that retains skin detail even when heavily manipulated by a modern portrait post program effects so popular with clients these days. While it may not show up in sub 1 meg compressed sRGB web jpegs, the large prints are stunning. Here are a few I just did for a wedding client last week that were very time constrained, including a candid AF shot (triptych below):
- Marc