If I had it to do over again, I'd buy aluminum. They cost more but they sure do hold up better. My 3-horse slant is a rusty mess already because I don't have a shed for it. It's even rusted through in places. The next one I get will be aluminum.
It's true the aluminum ones do not rust, but they do get pitted. Especially in the winters here. They are not maintenance free. We tried to clean the trailer the kids use and it looks the same. Of course the smooth side ones would be a lot better to maintain. The steel manufactures use better technology now and steel products do not rust like in days past. Look at how cars hold up better than they did 20 years ago.