I just just just received my Pentax 645N last night and loaded some film. Last night I'd downloaded and printed out the Pentax Manual and it's interesting in the youtubes on operating the camera that some folks admit they don't how to do X, Y or Z... or haven't used certain functions. Surely experience tells you how to work most cameras. Duh. But somethings are missed, and as much as I've seen it written that the manual is useless, there's stuff in there that can be helpful with every camera that's the builder's often idiosyncratic way of allowing x, y or z within their design goals. There's more that only use focuses your attention on in terms of "how do I ______" or "what's this mystery button?" but I'm not there yet.
But for one, the oft stated "can't change films" is true in terms of using a film back to do so; however, Pentax (and I think at least the Nikon F100, too) has set up to provide a motorized rewind of the film back on to the starting roll should the photographer wish to substitute film inserts. Surely this would be awkward and fraught... but it is doable. You'd have to keep meticulous notes, too, as to last frame shot and be sure to re-advance to the first unshot frame on re-load, so that's not going to be something that's nearly as easy, but it's still do-able. And while in my case, I just don't find this all that limiting, I'll admit that when I load my own 35mm cartridges with bulk film, I prefer to roll 24 shots tops if I can.... because lower numbers work better for me.
As to the motor advance, one of my former cameras - a Rollieflex 6008, had motor advance that by comparison makes this sound positively quiet. The Rollieflex 6008 though did have that oh-so-German shutter sound that could have been the inspiration for Mercedes door "thunk".... and that was a marvel of its own.
Having done the 6X6 thing, this 645 frame size offers a smaller footprint no matter which you go with. The Mamiya 645 is sometimes called the RB67's smaller sibling... and I think it's similarly sized. Some have made the suggestion that the all-in-one Pentax 645N has some virtues (everyone might pick different ones) that the "system" approach can't offer - e.g. fixed, single box film path. But the Mamiya 645 lets you customize and go smaller with a manual wind. If I'd gone that way, that's what I'd do and skip the possible issues of a motorized wind. There's a lot to be said for simplicity in keeping electronics restricted to focus and exposure... and perhaps my acceptance of a motorized wind will prove an issue. Time will tell. But that said, I think MF in general (including digital Fuji's) benefits from a handle or wrist grip to help "the carry". Comparing the Pentax 645N to a Nikon F4, this Pentax MF camera body (without the lens) is virtually identical to the footprint of the F4 with an 85mm lens... But then you have to add a Pentax lens on the end of where the Nikon 85mm lens would stop. Width across the back where the film plane lies in the Pentax is slightly smaller than the Nikon, but the body depth from front to back exactly the same as my eyeballs measure it. Height of the Pentax and F4 are roughly the same.
In my case, either could work, and whatever floats your boat... "at least as best as you can do ...and do until 'better' comes along" will work." One thing I like about the Pentax 645N besides the fact that I already had an A-150mm on hand is that I came after the camera with a fairly clear purpose of keeping the shooting simple, and package of gear light-enough (it's NOT lightweight) while still getting a larger than 35mm frame. Also, I liked that Pentax still uses this form factor in the digital world and the lenses could be used on an actively manufactured camera without an adaptor (I've tried using the 150mm on a GFX with an adaptor but the bulk and added weight just ain't worth it). That said, this potential crossover use was NOT NOT NOT a decision point so much as a "could be". in the back of my mind. Finally, my first digital camera was a Pentax some years back... and news of their new Monochrome as well as consideration of re-entering film photography spiked enough of my curiousity to add some intangibles to the mix.
Would I like a 6X7 frame size? Sure, but that's not nearly as portable... and recognizing that is a matter of experience and pushing that "bigger frame size" up to a "yes it ONLY sits on a tripod camera" (4X5) and either shooting sheet film or using a 6X9 roll back formed my decision.
Not sure any of this will help your decision, but perhaps. In any case, decide-buy-and-enjoy! Have fun.