I just left a similar post on another topic, but I have used all sorts of lenses--Hasselblad V, Hasselblad H, view camera lenses,as well as the Schneider and Rodenstock VC lenses and more recently the Schneider and Rodenstock HR digital lenses--which I felt there was no equal to until starting this year to use the Phase One Camera in addition to my Alpa, and using the Schneider LS lenses--they are as good or better than anything I've ever used and would recommend them without exception. You really need this kind of quality with the high resolution back you are using--anything less will be distinguishable. I'm using the P65+ loaner with these lenses (waiting for my IQ180 arriving this week) and I print my images about 40x60-70", so I'm tuned into high resolution. I have not used any of the Mamiya lenses on my Phase One, but hear they're very high quality digital lenses also, but for my money the Schneider LS lenses are the way to go with a HR DB like you have.
Dennis