2. I often use a 70-200 on my Canon gear
If you often use the 200mm end of that lens, that could be a strong argument for staying with Canon, or at least keeping it for telephoto work.
Medium format has never catered much to telephoto, not nearly as much as high end smaller SLR format systems like 35mm do. Mamiya can go beyond 300mm, but requiring distinctly higher higher minimum f-stops than f/2.8, and thus a need for higher ISO speeds to maintain adequate shutter speed in most telephoto work. [A 2x tele-converter requires quadrupling the ISO speed to maintain the same shutter speed.] So it seems unlikely that there will be much gain in image quality: high ISO is not a strength of MF, with either film or digital.
The exception is if you anticipate being able to use rather low ISO speeds despite the combination of higher f-stop and long focal lengths.