I have two Mamiya 645 bodies - a Super and a Pro.
I like what I have, and prefer them to the older M645 bodies, primarily because of the interchangeable backs, but also because I prefer their weight, and the more modern prisms.
In my mind, the biggest reason to choose the Bronica cameras is the fact that they use leaf shutters in the lenses. On the other hand, there are also good reasons to prefer a focal plane shutter (consistency of exposure, lower weight and simplicity of construction of lenses).
There is a lot of Mamiya equipment available on the used market. My perception is that it is harder to find Bronica, Pentax and Contax (in that order).
I find the ergonomics of my Mamiya equipment to be excellent. There are some minor irritations (like the proprietary straps, for instance) but I think that ergonomics is something quite personal, so I would strongly suggest handling the various options before buying.
FWIW, here is an image I shot handheld using my Pro, with the 55mm lens. That lens is very good, and very reasonable.