Just a .02 add on...
First off, congrats on your system and I think you made an excellent choice. The Hassy cameras are excellent and the current H3D2 31 promo pricing is hard to ignore. Second, you make good point about the perceived quality -- I agree and I shoot the Mamiya
But, what I've found since owning and using the Mamiya: The newest lenses are built with a better feel, metal barrels and nice rings, and rival Hassy -- both feel and look substantial as they should. Quite frankly I also expected sub-par performance from the plasticy-built Mamiya primes, but have been pleasantly surprised by outstanding optical performance, even wide open! The next thing is while the Mamiya AF may be noisy, it is fast (AFD2 body) by MF standards and maybe even faster than Hassy. Plus it works well in lower levels of light than other MF AF cameras I compared it to... The new Phase or AFD3 body that is due out in a few weeks promises to improve on this further, but I still need to see it to believe it.
That out of the way, I think the main difference to look at is owning and using a leaf-shutter system versus a focal-plane shutter system. In the field (I primarily shoot landscape) the higher shutter speeds of the focal plane system are often needed; the same light would require adding ND filters over a LS lens. Yet in the studio, higher flash synch speeds of LS are certainly more desirable. Also, a focal-plane system like the Mamiya does allow for mounting of many older manual lenses -- I routinely use a Hassy 110 F2 and several older Mamiya manual focus lenses on my Mamiya AFD2. The other thing the new AFD3 body promises is LS lens compatibility. When Mamiya brings out LS lenses with flash synchs at 1/500th or higher, Mamiya becomes a pretty flexible competitor I think. There is also a rumor that Hassy is coming out with a focal-plane body that uses their current lenses, so some really nice flexibility on the horizon there too!
Interesting times for sure, and it is nice we have so many really good options...
PS: As respects the backs, IMO they are all very good and one can hardly go wrong choosing any of them. Most folks probably choose based on camera compatibility, UI, workflow and speed/ISO performance than over differences in the files themselves.
Cheers,