But regardless of the camera that I select, do you think all of these cameras are fairly reliable and robust? My thinking is that the Mamiya can take the most beating and is a better workhorse. I hear the plaubel is built well but the internals are quite fragile.

I'd rather go 6x6 so a Mamiya 6 makes more sense if this is a main shooter. I wanted a Fuji 645 because i figured that I can sacrifice the format for compactness. I think Mamiya 6 and Fuji 645 make a more sensible combination: 6 as the main shooter and 645 when I really want something compact. If I had to choose one, then i think it would be the 6 at the moment.