Corrugated or smooth outer finish?

I think you're somewhat correct. A corrugated side wall will tend to shed more water dripping from the extrusions on the upper part of the sidewall by dripping. Unfortunately, the aluminum sided trailers will also develop black streaks if organic matter is left on the top and rain occurs.

Since you have covered storage, there shouldn't be too much of an issue with streaks for either choice, as long as you park with a somewhat clean roof.