I have a fairly nice Pentax 645N system, with 9 lenses. I love the system. Having few opportunities for trekking, if any, and finding the camera with 3 lenses already heavy for a daily carry when working (so not using a backpack), I bought a Plaubel Makina 67 with the Nikkor 2.8/80mm. Ever since, I'm shooting more, always taking the Plaubel along wherever I go. I had to get used to focusing, but sort of like the different method of framing, and adore the detail I get out of it on 6x7.
My only real regret is that it's not for portraits (and I don't think the Mamiya 6 and 7 are, either). But then, I have a system that really is (Pentax 645N with either 135 or 150mm). The Mamiya 7 has a reputation for sharpness that is apparently unbeatable. If weight is a concern, every ounce counts. I'd go for the Mamiya if I wanted more lenses. If only one, you have more choice.