I have both the Fuji 645zi and the Pentax 645 (not N). The Fuji weighs 2 pounds, and the Pentax weighs more, enough to be a bother. Both of the motor winders make noise. The Fuji takes 1sec to focus, while the Pentax is full manual, and for me a little more flexible. The Fuji is a point-and-shoot on serious steroids, and the Pentax is an SLR with some beef. Both have really good lenses, but I haven't done a side-by-side test.
Originally Posted by puketronic
If getting the decisive moment is important, don't use an AF camera like the Fuji. If you don't mind going through a tad more work to make a photograph, take a look at using a TLR. I have a Yashica 635 (6x6 and 35mm) and it's compact and weighs the same as the Fuji 645zi.
I don't really have a definite favorite for MF cameras. They all have a niche.