Painted trailers are suited for freshwater use and are the lowest cost option of the three. Painted trailers can be more visually appealing than the galvanized trailers and in some cases will match the boat color.
Galvanized trailers are suited for salt water use as they offer extreme corrosion resistance. All surfaces of the trailer are coated (inside and out) as opposed to just exterior painted surfaces. Galvanizing will dull over time and may not be as visually appealing as painted or aluminum trailers. Galvanized trailers are a higher cost option than painted, but will always tend to last longer under normal conditions.
Aluminum trailers are suited for salt water use as they offer extreme corrosion resistance. Aluminum trailers are much lighter than the painted or galvanized counterparts reducing the overall load on the tow vehicle. In terms of appearance, aluminum is very appealing from the date of purchase and for years to come. In most cases aluminum trailers are the highest cost option.