IMO, the reason, and the only reason, the Pentax outdoes the Mamiya, is the 105mm f/2.4 lens, which is outstanding. It is significantly faster than any Mamiya offering. Otherwise, the Mamiya is a far more versatile camera in every way.
I thought I wanted a Pentax 67 as well when I had just started, but then I discovered RBs. In my experience, the people I know who prefer the Pentax are folks who are really pretty technically inept, intimidated by medium format, and prefer it's "big 35" look and feel. This in no way makes the Pentax a bad camera, nor does it mean these people don't produce outstanding work! There is just a bit of a different approach to using it, I think. Better suited for people who want the utmost in simplicity and ease of use, while the RB is better suited for those who are more interested in functionality, and versatility features.
Also, for some reason, the Pentaxes still seem a bit expensive on the used market, while RBs are going for dirt.
Re: "The RB67's are good cameras, too, but also heavy and boxy. These babies can't be handheld, IMO...maybe the RZ67."
Agreed in general, but they definitely can be hand held, as long as you can use a fast shutter speed and a short or normal lens. Same applies to the Pentax, but I would say it is even less hand holdable than the Mamiya with a WLF. Also, remember you can use MLU and the sportsfinder on the Mamiya, which can help.