I've bounced around photography systems a lot as well. My first medium format was a Mamiya 645proTL. While I had that, a teacher had the Mamiya 6 and we traded cameras for a half day. I sold the 645 soon after and went 6. I had to sell my first 6 during a rough patch 20 years ago and regretted it ever since. In the interim, I tried a Bronica RF645 (great lenses, but the vertical 645 was weird to me), A Mamiya 7II (I bought a "broken" one and had it repaired). I then sold that to fund something else (at about half the price that they are fetching today). I now own a slightly rough 6mf. I've been running some rolls through to check for any obvious issues, but I love it. It's the camera I am most comfortable with, and like people above have said, the 50mm is a dang fine lens.
I've handled a Plaubel PM67, and thought about sourcing one here and there, but never have. For me, I think what I have been trying to deny all these years is that I am a "square" guy.
I have never called an industry person to verify this, but the rumor mill states that the 6MF has an updated and/or strengthened film advance mechanism over the M6. You also get the multi-format frame lines to deal with in the range finder window. Give and take.