I vote for the Pentax. One great feature of this camera is that you can use Pentax 67 glass via a Pentax adapter. Works seamlessly.
I had the Mamiya 645 camera too, but found two things I did not like: (1) film back has a tendency to become loose; and (2) lenses were made of cheap plastic.
The downside to the Pentax is that you cannot change film mid-roll or attach a digital back. Other than this, the Pentax is a better design IMHO.