Both cameras will have there benefits and drawbacks.
I have the original 645 and the 645n. If you don't need AF the original 645 isn't any less of a camera than the 645n, though there are differences in the way they handle.
Benefits of the Pentax are, it's has motor drive and AE prism built in and is smaller than a similar equipped Mamiya 645. Both Pentax cameras have a standard threaded cable release socket, and the 645n also has a socket for the Pentax electronic Cable Switch F. One often overlooked feature that is (I think) unique to the Pentax 645 cameras, is that there are 2 tripod sockets. One on the bottom and one on the side for shooting vertically.
Drawbacks include, lack of mirror lock up, lack of changeable finders and film backs.
The Mamiya 645 system (I've never used one) is more of a full system, with changeable finders and film backs that can be changed mid-roll. The Mamiya has mirror lock up.
Drawbacks being, if you want motor drive and auto exposure, you need to the proper bits for those features. The Mamiya doesn't include (normally) a threaded cable release socket, you need to find the little adapter they made for using a threaded cable release. The Mamiya, once a motor drive and AE prism is larger than the Pentax.
Another option worth exploring is the Bronica ETRSI.
Here's a review of the system that I wrote,
Bronica ETR-Si review | Colton Allen | Film Shooters Collective