It just so happens.
I use both a Rapid M with a 90/3.5 (Hexanon) and a Mamiya Universal Press with the 100/3.5 and 90/3.5 with either Polaroids or a G-adapter.
Actually, I bought the Universal
after I managed to get a G adapter.
As it happens, I just started writing up my notes for my various systems and the Koni went first. The Universal Press is up second. My notes are geared to the more advanced, experienced user and don't really repeat what I've seen on the Internet. Not a review so much as a terse report. It's a first draft and may get a little more, but it makes all the important points. I'd post a link, but this is my first post so I can't. It shouldn't be hard to find, or someone else could post a link.
I'll add a bit more here, because I was going to put this in the Mamiya page.
In any case, I like the Koni better as a tool, but the Mamiya is more useful, especially for 6x9, for useful interchangeable backs, its 100/2.8 lens, and its uniquely large 3x4 instant film options. The Universal sees far more use; though I don't like rangefinders, it probably is my most commonly used film camera.
I should point out that the Universal and the TLR C-series have very common lens mount setups and it could be possible to "expand" the choice of lenses.
In the end, they weigh about the same, but the Koni just feels much heavier in the hand. Mamiya packs far better, especially with a 90/3.5 collapsible lens and smaller backs.
The Universal grip does not agree with my style. I made a quick release right hand version that clamps to an arca plate. That said, I usually use it without either grip.
One more thing. If you can, get a Mamiya where the very thin faceplate around the VF/RF windows is held in place by screws. You will need to remove this plate at some point to clean up the optics when they get fogged. The screws are a lot more fun than prying it off without tearing it and then gluing metal back - but not permanently - into place.
Both are excellent and reliable cameras.