IMO, it is mostly due to scratches/dings. The white will show a scratch or ding worse than the extruded. The extruded is a little more resistance to scratches/dings and can be buffed out easier. The extruded also adds a little more stability to the frame/axle box assemblies as it is slightly stonger than the white sheets in most cases.

We have acid and pressured washed ours for years and have absolutely no issues. They use the white skin on over the road semi trailers for years, and acid wash these a lot-I have never heard of any issues.

The white aluminum sheeting is .040" -.050" thick, and on the newer trailers, held on by tape. The better newer trailers will have the panels' perimeter also secured with rivets, to eliminate the eventual lifting of the edges of the sheet metal. The tape allows a slight movement of the sheeting over the structure, so the whole structure isn't improved with much additional rigidity as a result. On older trailers, the sheeting is rivited along each stud and most horizontal framing. This greatly enhances the over all strength of the walls.

The paint is quite sturdy considering its very thin application. It is applied in rolling mills and baked on at a high temperature. It can be damaged by mechanical means and constant abrasion, which will show the base metal underneath. It should not be acid washed. If you are determined to use a truck stop facility, have it citric washed instead. Most automotive products work quite well to clean and polish this finish.

Extruded siding is ~.125", and most pieces interlock to the lower one. Most importantly, each piece is individually welded to each wall stud. This results in a very strong wall unit. The additional thickness also eliminates many of the impact damages, commonly found on smooth sided trailers. The metal is solid, so more serious scratches can removed by sanding and polishing.

An extruded siding trailer is more durable, will show less damage, and is easier to keep clean.