Aluminum...a buddy of mine who was a transporter for many years - millions of miles - bought a Featherlight enclosed many years ago.
Cost has never been an object for him, and he was looking for the weight advantage.
He took really good care of his stuff, religious on maintenance and the like.
It cracked, IIRC, near the tongue. He went back to steel frames.
I know there are a lot of them out there, and they have their fans, but that was his experience