As a steel fabricator I wouldn't consider aluminum unless weight was the overriding factor.
Aluminum doesn't give or bend as much as steel, meaning it's more prone to breaking out right. It also doesn't absorb vibrations as well as steel, which is not good for trailers that are subject to constant dynamic loading going down the road.
This is all spot on, speaking as a Certified 3G alum welder for over 40 years. I own a open Featherlight I mentioned above. If you plan to use on a regular basis, IMO the first thing to look for on an Alum trailer is ANY structural weld. The less welding the better, the location is also critical, they all will eventually fail based on time of use, and loading. It's the nature of the beast. Seems to me the most problem prone area is where the A frame tongue gets welded to the rectangular flat car carrying platform. The less welding there the better. Don't forget all welding on alum only make the alum weaker, and the weld causes a stress concentration in all situations. Featherlite has been the best execution I have seen so far.
All this said, I would never own a steel one car carrier trailer.
This is what I design, fabricate, weld and install in Alum for over 4 decades.
