The primary difference when I bought mine was cost, I was able to get started in the Mamiya 645 system for less money. Once there, I stayed there and upgraded components. The Pentax body is all in one piece, so you have your prism, winder, body and back together. Since I am buying used stuff and one concern is breakdown/repair cost, I find that the modular system used by Mamiya is more flexible. I have had a body go out on me and a replacement body alone was about $150, it would probably be less now.
Originally Posted by game
Another difference is that if you like to have film ready to go, the inserts for the Pentax are much more expensive than the Mamiya inserts. With the Mamiya, a couple of spare inserts for additional film might cost you $20 to $30 each and the Pentax inserts will probably be over $100. The same issue comes up if you want to use 220 film. With Mamiya you can get a 220 insert for $20 to $30, with Pentax it will probably run closer to $100.
Also, if you are interested in using a waist level finder, you can get one for Mamiya and not for Pentax. In my quick survey or KEH, Pentax lenses seem to be a tad more expensive as well.
On the other hand, at least at KEH, a Mamiya 645 Super body, back, AE prism, and winder, will probably cost more than the Pentax body, which has all of the other pieces built in.
I firmly believe that both camera systems will give you great images and are of very high quality, but those are some more difference that I know of. Again, I have not used the Pentax, but these are the things that I considered that led me to buy the Mamiya given my situation.