I'm an actual user of a H4D-50. I see advantages in the integrated approach adopted by Hasselblad.
Perhaps the difference between Phase and Hasselblad is that Phase One are primarily makers of digital backs, whereas Hasselblad are primarily makers of Cameras. That may be too simplistic, but it does inform their design approach. Hasselblad have gone for the integrated approach where the digital back is but one part of the bigger picture. This allows them to offer True Focus better system integration.
I have been and remain happy with my H4D-50 and would recommend Hasselblad. With Hasselblad you are buying a system, not a component in a system.
Yes, I agree that is too simplistic to consider Phase One a maker of digital backs, considering that in addition to making digital backs, Phase One also has a dominant ownership position with Mamiya, and counts in its product line Leaf Imaging, who have their own unique digital back technology as well as 6x6 camera technology (via the AFi). In addition, Phase One has forged a long term, formalized partnership with Schneider Optics who, in a relatively short period of time, have announced 6 Leaf Shutter lenses (4 currently shipping), and a 120mm T/S lens. This doesn't even touch on the software side of their business with Media Pro and Capture One.
The primary difference between the approach of Hasselblad and Phase One in terms of camera systems, is the level of integration and the fact that while their is indeed integration between the Phase One and Leaf digital backs and the Phase One/Mamiya DF Camera, Phase One/Mamiya have not restricted the ability of 3rd parties to create a digital back end for their front end product.While currently selling options for that are scarce, there are still a very large number of 2nd, 3rd, 4th generation digital backs that still have that option. The level of integration is certainly different, and that is due to the very different electronic design starting points of each camera.
Considering that there are not one, but two new cameras in development by Phase One/Mamiya, combined with the Schneider partnership, and factoring the Leaf technology team, I'd say Phase One is anything but primarily a digital back maker.
So back to the original OP, who probably fled for good reason, it is likely that his choice of H4D-40 and P45+ were based on pricing similarities, and as such, certainly there is merit in comparing the two products. Since he did not embellish his intended usage, the positives of each system stand alone.
*Best long, long exposure solution on the market for large sensors and high resolution
*Close to 90% 645 frame coverage
*Lower ISO starting point
*Significantly superior software platform with Capture One
*Ability to be used on other medium format camera platforms
*Ability to be used on technical/view cameras with a very wide array of the sharpest lenses available
Perhaps one of the most important unstated advantages for a Phase One owner is investing into a company that is internally-owned, with no one to answer to but themselves and who, from any objective accounting, are thriving as a company that possesses a desirable collection of segment-leading technology with a robust R&D department. And based on what we saw this year with the introduction of the IQ series, there's a lot more to come.