Plastic will scratch. But once in the water they will be non relevant.
- and you can polish them out if needed
Glass is obviously stronger, but once scratched, that's it :-(
Glass has better water dispersion. (I believe) So if you want to do lots of above/below spilt shots, glass would be better.
These are points I've gleaned from wetpixel etc. There are many on here with vastly more experience in these things, why not give a call to ReefPhoto or similar. When I've needed advice, the guys there have been exceptional.
but I've always stuck to glass, to me having plastic lenses just doesn't seem right :-)
F.I.T also do a glass dome for the 8mm on the Mirrorless mount.
F.I.T. 4.33'' Optical Glass Dome Port for Nauticam M43 MILC Housing