I have plastic--never used galvanized--and think I will stick with it given our current circumstances. One benefit to the plastic over the galvanized is that it is thermodynamically more stable than the galvanized. Not to say that it won't heat up or cool down to temps the galvanized will, but that it will get there more slowly. Galvanized will also transfer the heat into or out of the water more quickly and to a greater degree than the plastic will. I will concede that it might be a difference without a distinction in some circumstances. 101* ambient air temps are just hot regardless of the waterer's material. And likewise, temps in the teens will just as easily freeze water in the plastic one as it will in the galvanized one. But in more moderate temps, it will translate into more even water temps across a given time.
I have two waterers and rotate them daily. On the "off" day, one is getting washed, so cleanliness isn't so much of a problem.