I have a metered prism 6x7 with MLU ... wow, it's a brute of a camera. I find it heavy and not very ergonomic. But ... every time I develop my neg's I'm amazed at the picture quality. The 105mm F2.8 lens is an especially wonderful piece of glass. Excruciatingly sharp with wonderful, fluid bokeh.
The Mamiya7 is also a nice unit (never owned one) but it's probably considerably more expensive than a decent Pentax 6x7 + lens.
I've handled a Mamiya Super Press ... I'd dare say it's even bigger than a Pentax 6x7. It has the advantages of interchangeable film masks for 6x6 6x7 and 6x9 if I remember correctly. The lenses are supposed to be pretty good, but I've never heard anyone rave about them. These cameras are getting harder to find it seems, although they pop up on fleabay. My local antique camera vendor has one on his shelf right now, he wants $290 (Canadian), with a 90mm lens, I think. So they're cheaper than a Pentax 6x7, but you don't get metering or the same flexibility in lenses.