I'm way out of date on Mamiya 645, but I'm assuming that since you are planning to use a digital back you're considering the AF body and lenses. Our experience was with the manual focus versions of the 150 and 45 (or was it a 50?). Ours were superb performers, and I have to assume that if Mamiya intended the AF successors to be acceptable to established customers, they also would be up to snuff.
BTW- The manual versions have leaf shutters, which were really useful not only for landscape, but also for very high flash sync (>1/500) to freeze very fast motion. If that kind of shooting is within your realm, I'd check on the leaf shutter feature. We didn't realize how much we would use it till we had already bought the gear.