Every one of the cameras mentioned have great reputations. You really can't lose with any of them.

I have a couple of older, first edition Pentax 645's. These cameras do have some limitations, particularly in the area of interchangeable backs and finders and for flash shooters. None of them mean much to me so I don't feel limited. The upside of the P645 is that the older, non-AF bodies are available cheap and the lenses are pretty much all outstanding and, due to their lack of shutter mechanisms, they are relatively cheap as well.

The 6x7 will have better negative quality due to size but 645 cameras are smaller and lighter in weight than the 6x7's. I consider my P645's to be oversized 35mm SLR's in handling. The negative quality is every bit as good as a cropped 6x6 and considerably better than a full-frame 35mm.

Both the Pentax 67 and Pentax 645 (and probably the Mamiya 645 as well--I'm not familiar with the system) have a selection of long lenses available but they're expensive, big and, of course, they don't have the magnification of 35mm telephotos. Medium format, at least in my opinion, works better with shorter focal lengths.