I have used Mamiya 6's for about two years now, I have no experience with the Bronica, although it has alot of nice features.
Bear in mind that the "default" framing on the Bronica is Portrait format (i.e., when holding the camera "normally" the picture will be taller than wider) - I JUST KNOW THAT THIS WOULD DRIVE ME CRAZY IF I HAD THE BRONICA - especially if I wasn't shooting mainly portraits. With 35mm, I would guess that more than 90% of my pictures are in Landscape orientation, so the Bronica just would be too ackward for me to use regularly.
I would be interested in hearing the experiences of some Bronica users in this regard.
I've been very happy with the Mamiya 6 - I have never had a technical problems with mine, except I don't entirely trust the metering system, it's very susceptible to underexposure due to sky in the frame, so although I use mine on Automatic, I often carry a handheld meter to check, or meter more off the ground and use the AE lock.
As for the Mamiya lenses, in Medium Format I've used a Zeiss Super Ikonta, and Brooks Veriwide 100, Mamiya 645's, a Rollei TLR with 80mm Planar 2.8 lens, and the Pentax 67 system with only the 105mm lens.
Keeping the Pentax and Veriwide aside, I would rate the Rolleiflex and the Mamiya 6 "the best optically" out of these, but in entirely different ways. For me, the rollei is about smooooth bokeh, and lovely rendition in Black and white, while the Mamiya 6 75mm and 50mm are very sharp and render colours better. These lenses only have an f/4 or so maximum aperture, so they're not fantastic for low light situations.
The bokeh on the Mamiya 75mm and 50mm is "okay", but the 150mm has FANTASTIC bokeh and despite only focusing down to 1.6 meters or so, is not so bad for portraits which include the upper body - I usually shoot it at minimum focusing distance at around f/8. I find the 150mm too big to lug around, so it doesn't get so much use.
My current favorite lens for the Mamiya 6 is the 50mm though. It's nice to have a bit more depth of field for fast moving shots, and there's just something "about" that lens.
'Hope this helps in your decision making process.