I use both and prefer the plastic only because the galvanized ones do tend to look crappy once the coating wears off and they could rust in some spots.
As far as the plastic ones, I ONLY buy the ones that do not leak (if not kept absolutely level).
I check to see how the plastic reservoir screws onto the base. If it "locks" into tabs underneath, I don't buy it because they leak.
If there are NO openings or tabs, I buy those (they simply screw on).
I've purchased enough to know that leaking waterers SUCK!
Also,
I cut out gallon plastic water jugs (since we buy a lot of distilled water) and use those as waterers (and feeders).
When these get dirty or unusable, I just toss 'em.
The galvanized waterers are good if you plan to set it on top of some sort of heater (I use a light bulb inside the 'hole' of a cement block and put the waterer on top of the block).