As has been mentioned all have their advantages & disadvantages. Go with what you like & can afford.
The corrugated aluminum is probably the cheapest but easiest to repair if there is a fender bender. This type have been around over 60 years.
The flat laminated sides have been around probably 20 plus years. These can delaminate & are harder to repair if in a fender bender, but look nice & easier to wax.
The air stream type been around longer but too are harder to repair & if scraped on a tree are hard to repair always will show it. But very well built trailers. Probably more old Airstreams around than any other brand.