Hello,
I've used the Mamiya 6, 7ii and the Plaubel 67, and my 2 cents would be:
They all take fantastic pictures. They all cost real money. It's mainly in the ergonomics, functions and lenses that you make your choice.
RF spots and frame lines on the Mamiyas are much (much) brighter and the Mamiya RF seems way more accurate. Also meter readout is much easier to see in the Mamiyas than the Plaubel.
The Mamiyas have aperture-priority auto, so can work much faster if you like that function ( I do).
Focus by thumbwheel on the Plaubel is weird and slow, at least for me. I also use old RF folders that focus this way, so it doesn't confuse me too much (it might confuse others - takes getting used to and you may find yourself adjusting aperture or shutter when you mean to focus) it's just that it's actually, physically hard to use, compared to say old Voigtlanders which are very smooth.
Having both speed and aperture on the Plaubel lens is OK if you're used to old cameras; it may confuse you otherwise or slow you down.
Obviously, the Plaubel only gives you one lens. Which is fine, but the Mamiyas are complete systems. Even the 6 with only three lenses pretty much covers everything. And all three lenses are basically perfect.
I usually like square more than rectangular, so prefer the M6, which is also (to me) the most comfortble to hold and most ergonomic and logically laid out.
M6 feels way more solid than M7ii. The fact that it collapses is huge - much more portable.
Plaubel looks way cool and minimalist, but it's actually kind of hard to hold, at least compared to the Mamiyas, which are molded to your hand.
M7/7ii needs stupid external finder for its wide lenses while the M6 has all frame lines built in. This is a big deal for me - I hate bobbing between windows like a chicken (and I hate having to carry these things around or worse, forget them..big problem!) Whole point of these cameras is they are easy to carry and work fast.
M6 uses same lens back caps for all lenses. M7 uses different ones. God help you if you lose the back cap for your 50 or 43mm lenses. Clearly not an issue for the Plaubel.
Mamiya lens hoods are stupid - while on, it's nearly impossible to put lens cap on or off (unless you have lots of filters stacked on). Not a big deal if you use UV filters instead of lens caps (actually a good idea for most RF's..fewer black frames).
Mamiyas allow you to dial in up to 2 stops exposure control directly and easily seen on TOP of the camera around shutter speed dial. Plaubel requires you to calculate in your head or move ASA ring..which is on the BOTTOM of the camera, and easy to forget about.
Speaking of which, the ASA ring can be easily, inadvertently moved while adjusting shutter speed, whic is bad.
I LOVE multiple exposures, and that was the one thing that kept me with the 7ii so long, though I much prefer to shoot the 6. I recently sold the 7 since I didn't use it much, but regret it, and will certainly get another, just for that reason. If the 6 had (or could be tweaked to have) multiple exposures it would be the best camera ever made, for me.
Repair on the PLaubel will cost much more than on M7ii.
Plaubel's design is more prone to problems - i.e. the wires connecting the lens to meter readout, lazy tongs more likely to go out of parallel. Also, you have a bellows you can puncture (though I haven't had that happen..but no chance on the Mamiyas).
Film box top holder on the Mamiyas are on the back and the right size- on the Plaubel, it's on the side of lens mount and too small! Means you have to trim the box top, can't see it when camera is folded down, and it can fall out inside! Detail, but still.
The Mamiyas are much quieter than the Plaubel.
However, if the batteries on the Mamiyas go, your shutter dies. Not so on Plaubel.
There may be one or two other things I can't think of now...
Best of luck
