Based on my experiences so far with both kinds of waterers I would only recommend using the 1 gallon plastic waterer, and anything bigger than that go galvanized. I have seen many people buy "chick waterers" that are less than 1 gallon (typically 1 quart); some being designed for mason jars and some setups that can be feeders or waterers--these are all very cute, but not very functional. Chicks can easily drink out of a 1 gallon waterer so there is no need to get less, because you never have the utility with anything less. Only on rare occasions will you be able to use the 1 quart waterer after your chicks are grown, so why bother?
One of the problems with prepper and survival sites is that what works in one area or situation will not always work in another. Climate is a big issue. I have only used one heated plastic waterer and it did not work. Galvanized waterers have what seems to be a good heated base, but I have never used it. I could see many situations where using only plastic would be better than galvanized (assuming the waterers actually worked). If you need to give your critters meds, it would be better to use plastic than galvanized. That is something I have never had to do. I never even used the electrolytes either, which many people use. Eventually, though, you will start looking at easier ways to water your critters and that involves making your own automatic waterer, a subject for other posts.