clay or plastic reminds me of the age old question Ginger Or Mary Ann?
you get most of your plants in plastic bec its cheap. clay is heavy to ship so you dont usually get plants in clay.
I prefer clay over plastic but I do use the plastic pots inside the clay pots on occasion.
for the few oncidium types I have they are in moss and clay.
they live in a greenhouse and dont have any light, humidity, or watering troubles either so the moss and clay work. Its easier to over-water your plants in plastic bec the clay picks off a lot of the excess that stays around the roots in plastic.