I have been using Mamiya 645 for some time. Yesterday I received a Mamya RB67 Pro S with four lenses, 65mm, 90mm, 127mm & 180mm along with a grip, some filters and three film backs. I have shot 1 roll of 120 B&W
The 645 1000s with 40mm on the left, and the RB67 with 65mm right.
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Both cameras are a bit difficult to hand hold. Adding a grip makes either camera much more manageable.
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The same grip fits both cameras.
The 645 is simpler in use. Winding the film cocks the shutter, so it is meter, focus and shoot.
The 67 has a film wind lever and a shutter cocking lever. Both have to be operated to take a photo. As the bellows is extended, the exposure has to be increased by 1/2 to 1 stop. There is a scale on the camera for reference. Lens and film back installation are more complicated. Download PDF files of the manuals and keep them on your phone or some portable reader.
The larger image means more film and processing. DIY developing is cost effective. For a small initial investment you can process B&W as well as color film.
I hope this helps,
Phil