In the film days, 6x6, the classic was 50, 80 & 150. I mainly do studio portraiture and fashion. I use both the 120 and 150 on a H5D-50
The beauty of MF to me (not everyone) is the shallow DOF and the smooth Bokeh. So it's nice to shoot nearly wide open for some stuff. The focal length increasing tubes don't allow for that. Of course, if you are shooting at 5.6 - 8, that's no reason not to use them.
MF also wins hands down over 35mm in that I can focus manually through a bright, large view finder. So a fast lens is helpful, like the 150.
Sharp veins in the eyeballs are not important to me either, so I have successfully shot wide open at f4 on the 120 with manual focus. Slightly out but damned close.
The 150 won't allow close ups like the attached file taken with the 120. The 120 also seem a more 'intimate lens, like the 80 but with out the perspective.
My studio is only 6 metres, so it's awkward to get a full length with the 150 sometimes.
The 120 and 150 are my daily lenses. For me, they are neck and neck.
But all in all, I'd go for the 150 over the 120.