Back to the original qu: "PhaseOne P30+ vs Hasselblad H4D-31"
They use the same Kodak microlensed sensor, so they should perform identically on noise at a given ISO...with one exception: long exposures. PhaseOne optimised their Kodak-sensored P+ backs for exposures up to 1 hour, whereas AFAIK Hasselblad top out at the usual 1 minute.
Another (and for me, the biggest) "win" for the Mamiya/Phase platform is the sheer range of lenses you can use on it - thanks to two factors: its focal plane shutter, and having one of the shortest flange registration distances in medium format SLRs. With the Hasselblad H bodies, you are limited to H and V series (leaf-shutter only) lenses. With the Mamiya/Phase, you have:
* Mamiya/Phase 645AF lenses
* Mamiya 645 manual lenses (including exotic and fast ones like the 24/4 fisheye, 50/4 shift, 80/1.9, 140/4 soft focus, 200/2.8 APO, 300/2.8 APO, 500/8 mirror, and many more)
* Hasselblad V lenses (INCLUDING the F series like the 110/2, that the Hasselblad H bodies can't use)
* Bronica SQ lenses
* Pentax 67 lenses (attractive long, fast teles like the 400/4 EDIF)
* Pentacon Six, Exakta 66 and Kiev 60 lenses (Schneider, Carl Zeiss Jena, Hartblei, Arsat fisheye, etc.)
The point about the lenses is that the range is huge, the speeds fast, the cost low, and the niches are all filled. You cannot say the same about your options with the Hasselblad. Of course that may not matter to you, if your work only requires a limited setup like a 3-lens wide-normal-short tele kit.