Jonathan, that was exactly my position when I was shopping for medium format. I was inclined toward the Pentax, but the RB was clearly in the running. I was familiar with 35mm Pentaxes, and the 6x7 was essentially the same layout and would be used in the same way.
The Mamiya ergonomics are quite different, its strengths in use are different. Its more of a tripod camera, more of a studio camera, so I had to think about what I wanted to do in medium format. So, I found my way to a local camera store and arranged a brief tryout with the Mamiya... bought a roll of film, learned how to load it, put it on a tripod and took it out onto the sidewalk in front and tried to do a wide variety of types of shots in 10 exposures.
Then I thought about it while I admired the processed prints. I am not a studio photographer; I like to be out in the field, mostly hand-held. I have a real aversion to waist-level finders, having used a variety of them before. Like Hasselblads and others, the Mamiya can be fitted with an eye-level prism, but the cameras are just not shaped to be used comfortably that way. With its rotating back, the Mamiya RB is large and bulky.
The Pentax isn't exactly tiny either, but it's smaller. While it can be fitted with a waist-level flip-up finder, I think this configuration is awkward. It's clearly designed for eye-level handling, like a bigger 35mm SLR. The big Pentax is a superb field camera. Ultimately, that made the difference for me, and I knew I could be comfortable using it.
I found a pro realigning his gear, and bought his used P6x7. This was 1976, the year Pentax introduced the new mirror lock-up feature, but mine predated this. That's ok, I never missed the MLU. My next step was a meter prism and 200mm lens.
So, that was my decision, based on my preferences at the time. It was the right decision for me, which I later followed up with a second used 6x7 (with MLU), and then again later with a pair of 67II. Your preferences and priorities might well point you in a different but equally valid direction.