The Tamron 90mm macro is spectacular in the center (
see MTF), so I used it to check out the Mamiya 120mm M645 macro - said to be right there with the Hassi and Contax 120 macros, which shouldn't be as sharp as the Tamron in the center but then they don't need to be given the MUCH larger negative. Used a 40D and focused with LiveView, f/11.
Here's what I got:
Mamiya

Tamron

100% (I did have to shrink the 120mm image in PS a bit due to not getting the size quite right to match the 90mm):

Mamiya

Tamron

Overall they're pretty close, tough call on the quarter at 100%. On the penny it seems the Mamiya wins on the left side, but the Tamron definitely is better on the right side. Perhaps the DSLR body wasn't centered on the MF lens on my Cambo Ultima 35 test rig, since I just eyeballed the alignment and haven't made a proper mounting block for the 40D yet.
The Mamiya appears to have more contrast, but it's due to minor exposure differences. The blown-out highlights look similar, but are slightly smaller in the Mamiya shots.