Aluminum trailers are very popular here in coastal Florida. However, the cheaper ones are often made with a lot of galvanized cross members and plated steel bolts/U-bolts. The bolts get rusty and the galvanized parts cause corrosion of the aluminum where the dissimilar metals sandwich together.

There is nothing wrong with an All Galvanized trailer. They last a very long time (20+ years) when made right, and they often cost a lot less than Aluminum. Main frame should be made with all "Open" cross sections (like channel, not tube) so that all surfaces are galvanized. Tubular tongue and cross members should be open on both ends so it's galvanized on the inside.

Aluminum trailers when made properly with Stainless Steel hardware are top notch and weigh a lot less. Galvanized Torsion Spring axels last longer than Steel leaf springs if that is an option for you.

Both types of trailers will have items that need to be replaced every so-many years with a lot of salt use: Leaf springs (3-4 years), Steel Brakes (2-3 years), Stainless Steel Brakes (??years, but expensive), Steel Axels (5-10 years), Optional Torsion Spring Axels (7-10 years), Light Kits (3-10 years depending on quality), Galvanized Rims (10-12 years), Tires (6 years fresh or salt water).