As an owner of the 1.9 (non-N), I have to say that when wide open it is hard to focus because the DOF is +/- 1 inch! However, when I nail a photograph wide open, it sure is a joy. Is the extra stop worth the extra $, only you can decide that. I bet, both lenses at 2.8, of the same scene, wd be hard to distinguish
With respect to issues of handling and practicality:
1) The 2.8 lens uses the fairly standard 58mm filters, while the 1.9 version uses larger 67mm filters;
2) The 1.9 lens is larger and almost twice as heavy; and
3) The Mamiya 645 focus assist lever fits on the 2.8 lens but won't fit on the 1.9 lens.
I owned 80/1.9C for few years, it was a great lens wide open and it got better even closed down, I can only suspect N version would do better.
The only 2 reasons why I ditched all of my M645 gear were that I hated 6x4.5 framing and I got a nice offer to swap whole kit for even nicer SQ-A kit.