Which sort of macro imaging do you want to do most? Don't forget to have a look at the 60mm option too.
If you are shooting larger subjects/backing off more, then images will be softer with the 100mm as more water in between.
Glass or Plastic port, for macro really no difference.
100mm best option for small-tiny subjects and super macro work. 60mm great for larger subjets and temperate/murkier waters.
Can use teleconverter with older Nikon 60mm.