I own and frequently use the Bronica ETRs, Mamiya 645 Pro TL, and the Pentax 645, Pentax 645n and Pentax 67.
The Bronica and the Mamiya have the advantage of being fully modular, allowing you to switch film backs, grips and view finders (AE metered prism, manual prism, WL, etc.).
The strong points of the Mamiya are the wide selection and affordable prices of the lenses. In particular the 80/1.9 stands out as the fastest MF lens available. I also really like the 35mm. Furthermore, the powerdrive is readily available and cheap.
The advantages of the Bronica are the superb optics and the fact that they are leaf shutters, which is a real advantage for handheld work. It's hard to hold without the speed grip, so I would recommend getting one along with the AE-III metered finder for hand held work.
As for the Pentax, it isn't modular at all, doesn't have any particularly fast lenses, and uses a focal plane shutter. Nevertheless, the 645n is probably my favorite 645 camera! Why? It is the most comfortable in the hand, the optics are very good, the viewfinder is significantly brighter than any of the others, it has a focus confirm indicator, and I really like the data imprinting on the film. And the ergonomics are excellent. It is distinctly more modern than the others. And compared to the Pentax 645 (original) it is just better in every regard. Personally I wouldn't spend the extra money for the 645nii because the additional features are unimportant to me and come at a very high price. I only us A-series manual focus lenses with my 645n and they are wonderful. But if you really want the flexibility of a modular camera the Pentax might not be for you.
With regard to 6x7 cameras, you are right that the Pentax is quite heavy and not all that great for handheld work or portrait orientation. But it certainly makes a statement! If you want to shoot handheld 6x7 with an SLR I would strongly recommend the Bronica GS-1. It is a joy to use with the speed grip and metered AE prism.