For those that own the Phase One P30+ or 45+ or the H3DII-31 or 39 please chime in with some of the key factors that led to your final decision.
I haven't used either backs, but used both systems.
As far as the backs go though, are you aware that exposures longer than 32 sec are impossible with all the H3DII series cameras (including the 39 and the new 50)? This used to be a major strenght of Phaseone, although it seems that this is somewhat a thing of the past with their new 65+ super expensive back.
Regarding the systems, one potential advantage of the Mamiya system is that their 28 mm lens is supposed to cover full format, while the 28 mm from Hassy is limited to the current 36x48 mm sensors. I have been using this lens on my Mamiya ZD, and it is very sharp in the center, but soft in the very corners even stopped down. That is on a 36x48 mm sensor, I am not sure I want to look at the results with larger sensors. The really usable pixel count for high quality landscape prints with that lens will probably only be around 48 MP or so.
Weight is obviously a problem with the H system, but the leaf shutter lenses do IMHO provide a slight advantage in sharpness compared to the body shutter of the Mamiya. On the other hand Mamiya is rumoured to be about to release leaf shutter lenses themselves.
It is really a hard call whether less weight or lenses shutter will provide the best advantage when doing panoramic shooting with MF gear.
Finally, one aspect which is very rarely commented about in the MF world is real life battery life in cold weather. I have had comments from both Hassy and Phase shooters that it was OK, but nobody could really provide a clear figure about the number of frames than can be shot at -10C with both systems.
All in all, my personnal conclusion is that the best way to use these high resolutions back for landscape is probably on a pancake LF body like the Cambo WS, Alpa or new Sinar. Only those lenses can really do justice to the backs.
But then again, are these backs really worth it compared to the soon to be released D3x and Sony FF bodies? Only future will tell but I personnaly wouldn't invest in one without a prior good comparion with 35 mm FF. It really depends whether you want to stitch or not. If you belong to the stitching parts, then 35 mm FF is probably a much better deal for most landscape applications.