Hands down Pentax if you are comparing the older Manual Focus Mamiya to the more modern Pentax FA series.
I used the Pentax 645 35mm FA lens on a 1ds MKI, MKII and 5d MKII with the same adapter for years and loved it. I found the older
Mamiya MF lenses that allow for the manual aperture adjustments were not as sharp when shifted. You can easily get the full
18mm of shift with the zork and the Pentax 35mm FA. I would also stay away from the older Pentax 67 glass, I tried many of them
and they are fine on center but terrible when shifted.
Lenses I used with my Canon:
Pentax: 35mm FA *****
45mm FA **** not as good as the 35mm but much smaller and lighter to carry around
55mm MF **** older lens not the FA lens but very good.
150mm MF **** very old lens, but uber sharp. Only issue I had with it was color and contrast was not as good as with newer Pentax glass
Mamiya 35mm MF *** Great center on, but when shifted just wouldn't hold up on sharpness left you 6mm short 12mm being it's max
55mm MF**** Very good lens, not as good as it's newer AF version but still good to shift to around 16mm easily
80mm MF ***** Great lens, only one I had that would shift all the way to 18mm but when I moved to Pentax, I sold all the Mamiya glass.
Enjoy the adapter, I hope you have the newer style that now comes with the L bracket built it. Mine was much earlier and I had to modifiy mine to allow for an
L bracket. Loved the Zork. Good support from their U.S. rep at the time.