My choice would be, in fact was, the 150 mm lens. I have used both and find the CU adapter to be clumsy and while the framing would be more or less okay, parallax would provide unanticipated results. The price of the Mam 6 CU kludge is way out of line for what you get. I passed one up for $100 (argh! for the resale opportunity) but still think it is, at best, a compromise solution to getting way personal with a subject. If you do a lot of CU photography, you would do better to apply all that cash to an SLR, view camera or some other equipment that offers better utility for the purpose.
The 150 has its problems, too; squinty frame, slow focus, blah blah blah, all in the nature of RF machinery. However, I find it indispensible for scenics and medium range subjects, and a sharper lens would be hard to imagine.
I hope this helps, but please keep in mind it is only an opinion. I'm not sure there is a totally objective answer to this or most other questions where choice is involved.