I had good luck with my regular trailer with boards on top, lasted over 8 years and then I got rid of it as the rust was getting bad and I felt it was going to lose strength structurally soon. Never replaced the boards and the trailer is still in use today, different purpose but still being used daily more than a couple years later.
I thought about an aluminum trailer but nobody around here sells them and for prices I got that were more than double the prices of regular trailers, I decided to just use regular trailers, even with the salt air of the Gulf of Mexico making things rust fast (powder coat rusts just as fast as regular paint no matter what company applies it from around the country, the metal will still rust) so it is just a matter of time for metal down here.
You can spend a little bit more money for treated boards but I got over 8 years out of regular boards down here and they are still in good shape with the new owner so I am not sure treated boards is really worth it or not.