I have several MF cameras, and here are my thoughts. In the end though, it really comes down to what you want to do. I finally bought a Mamiya 7ii system after have gone through Pentacon Six and RB67Pro.
Pentacon Six, 6x6, Zeiss lens, very light. It's an SLR, I can't hand hold it that well. Besides that, I am tad bit weary of it's shutter speed. I had it CLA-ed when I first got it, but the guy told me, he fixed the shutter speed as much as he could, they are still bit off. I love this camera, awesome lens, esp for color. It's very cheap. Did I mention awesome lens? It even has an 80mm f2.8.
RB67Pro, polaroid back is awesome! It's a beast though. It's great if you are to set it on tripods. I know plenty ppl do carry it around, I tried, it doesn't work for me. Hiking sand dunes with that thing on your neck is a pain.
I finally arrived at the Mamiya 7ii system. So far, I love it. It's just like my Leica, but quieter shutters, and much cheaper. I only wish they had faster lenses for it, as the fastest lens is f4, since, I do most of my shooting at night. But with its electronic leaf shutter, I am pretty confident I can gain a stop just from that, which makes up some what for the lack of fast lenses. I haven't developed a roll from it yet, but from what I heard and saw on Flickr, its lenses are amazing.