I've been lucky to own all 3 (simultaneously!), obtained back when their prices were more reasonable, and I was single, working in the field with no expenses. That, along with GAS, was a perfect storm.
They all have quality lenses, but the Mamiya has telephoto options.
The 2 Plaubel's with the 80mm and 55mm is smaller, but not lighter than the Mamiya-7 with 2 lenses. The M-7 is probably more robust, but that's not saying much. The 43mm lens on the M-7 is the closes thing to the Hasselblad-SWC, only better. The Mamiya-6 is... well, square, not a 4:3. And the lens collapses into the body. A little, anyways. The Plaubels are not battery dependent. The Mamiya's have automatic exposure.
The format, and lens choice are probably more important to consider, that and the rangefinder experience. I much prefer SLR viewing, but you cannot discount to speed of rangefinder in the field, if that is important.
Note that the meter on the M-6 is center weighted and works well for shooting quickly, the M-7 has a weird off center spot meter that requires more care. The Plaubel is also a spot meter, but at least you know where the spot is.