I'm firmly in the "glass" camp. The humidity has nothing to do with what the enclosure is made of, but is the result of the amount of air-flow you allow.
More humidity, modify the opening. Need more heat in a tank? Insulate most of the sides & bottom...easy to do & many cheap ways to do it.

Glass allows me to see my pets & what's going on. I can add privacy to a glass tank, but adding windows to a tub? Not so good... Not a fan of plastic at all.