Let me throw more into growing comparison list. I have P645 and P645NII, Mamiya M645 1000, Bronica ETRSi. Do not own Mamiya Pro and later.
1. P645 comes with fixed prism, if you are not up to having only this sole viewing option P645 is out. But with that fixed prism it is beautifully compact. Pentax MF lenses are gems. The N (on top my earlier comments) add some bulk to everything including lenses and AF system is nothing to be proud of. But if N must be it ought to be NII with mirror lock up and also being youngest. But price goes up with every version, original P645 can be had for under $200 with standard lens. All Ps run with battery in.
2. Mamiya 645 1000: I like how it looks, it feels compact and solid, but once you start putting finders on it it's not so cute any more. P645 beats it in every way in my garage.
3. Bronica ETRSi is great, everything seems to fit nicely, yet it isn't even close to P645 in compactness and all-in body. Metering: got to get a finder, and the only one that actually makes sense is the expensive AEIII. For quick handling need a speed grip, which works great but changes overall bulk. The plus of speed grip is its manual operation, unlike P645 running on batteries.
For quietest operation Bronica is in front. Leaf shutter in lenses, manual advance, it all helps keeping it quiet. Can't stop P645 from whirling its way through the film after each frame and in the end. Shutter in Mamiya M645 is not that quiet, no idea how this works out on the Pro TL bodies.
I still prefer P645 possibly because it was my first quality MF system and got most lenses for it early on. And I love that external battery for P645.
But P645 has its electrical quirks as it aged. Mostly fixable, yet there and should be expected. Battery compartment, specifically battery holder, is not as durable as one in P645 N/NII, so all of it needs to be confirmed before purchase.