I went through a similar decision recently, and ended up with a Mamiya M645.
Like most, I started by looking at Hasselblads and Rollies. They were just way too expensive.
From 35mm, I'm used to shooting SLR with a prism, so decided to stick with SLR cameras. That dropped the possibilities to the following:
Bronica ETR family (ETR/ETRS/ETRSi),
Mamiya M645 family (M645/1000s/super/pro),
Bronica SQ family,
Mamiya RB/RZ,
Pentax 645.
After much teeth gnashing, I eventually decided against the larger ones (SQ and RB/RZ), because of the tiny number of frames one gets on a film with these cameras, the extra weight, and the added cost.
I think the 645 format cameras are especially cheap at the moment, as the wedding industry unloads their 645's to pay for 35mm DSLRs. That doesn't hold for the larger formats, which seem to be used more in studios.
Range of lenses for the Pentax knocked that off the list, so I was left with the rather predictable choice of Bronica ETR or Mamiya M645.
So then I ended up buying one of each, as I couldn't decide which I preferred. I bought a Mamiya 645 Super, and a Bronica ETRS. Either are great cameras. The Bronica is built like a battleship, and just oozes strength, but is a tad less advanced than the Mamiya. The fastest 80mm (standard) lens for the Bronica is f/2.8, whereas the Mamiya has an f/1.9. There seems to be a larger range of Mamiya lenses and other bits available, as well.
But the Mamiya feels a little cheesy in use. The body is plastic. Sure, the innards are metal, but it's just not built to last like the Bronica. Also, the focal plane shutter has a flash sync of 1/60, which is awfully slow. The Bronica leaf shutters flash sync at any speed.
So then I found an original Mamiya M645 with an 80mm f/1.9 lens. My plan was to buy the camera for the lens, and offload the body cheaply. But then I played with the body. The original M645, and the 1000s, is an all-metal body, built to last like the Bronica (actually a little better quality than the Bronica, I think). It takes all the Mamiya 645 lenses, and there are a range of prisms available (though one has to be careful when buying, as a lot of them suffer from degraded silvering). Disadvantages of the M645 are a 1/500 maximum shutter speed, the same focal plane shutter as the super, so the same flash sync issues, and no interchangeable backs.
None of these are a problem for me, as my shooting method is slow and deliberate, I rarely use flash, and I tend towards slower films. Should I want to shoot a brightly lit scene wide open, I'll just stick an ND filter on the lens.
Further, I figured the disadvantages of the M645 (mostly lack of interchangeable backs) mean that it's not beloved of wedding photographers, who flogg their gear to death. Despite this, they're still cheap, as people perceive them to be of less value than a super/pro, simply because they're older.
Sure enough, after cleaning up the M645 and buying a nice prism for it, I ended up selling the others.