I am new here and this is a very old thread, but I wanted to join this forum just to comment on this. I too have both the Mamiya 7 and the Bronica RF645. If you force me to choose one it would be the Bronica, hands down. The Mamiya 7 has glass that is second to none. The lenses are the sharpest I have ever used and I have used all the top Leica, Zeiss, Nikon, and other glass. I will state, howerver, that sharpness is not all that matters. Every lens has its own character and its own place in my heart.
The Mamiya 7, however, is by far the biggest pain in the butt camera I have ever owned. Everything is time consuming on it unless you only mount one lens and shoot it exclusively. Changing lenses is a chore. The crappy plastic dark slide key is super delicate and must be turned very slowly to avoid flexing and breaking it. The camera sometimes just stops working when it gets too cold. It is truly a camera to own only if you are willing to deal with all the BS to get absolutely stunning film images, which is delivers admirably.
The Bronica RF 645 is an absolute joy to shoot. It feels perfect in the hand. The darkslide is automatic. The program mode makes shooting a breeze. The settings are very easy to see in the finder and the rangefinder patch is super bright and contrasty. The lens quality is absolutely spectacular. They are not quite as sharp and contrasty as the Mamiya 7 lenses, but the extremely minor differences are easily compensated for in post processing. The images blow away my finest Leica and Nikon (or any 35mm) images to the point that I am literally angry that my $2000 Leica glass and $1600 Leica camera (I am talking used prices here) look positively muddy compared to the 645 shots. Although I have heard of some people having problems with the RF 645 I have had zero issues with mine, except the time I knocked the rangefinder out of alignment a slight bit when I got in a car accident and it fell to the car floor. The rangefinder is easily aligned with a tiny screwdriver through the hole near the viewfinder, however. It gives me 16 perfect shots every time, and is a breeze to load and shoot. I am truly saddened knowing that this fully electronic camera will someday be nearly unserviceable. The one thing the Mamiya 7 seems to have going for it is a fairly easy to source repair network...but you can be sure you will need to repair it.