After owning both I will only purchase fiberglass sided trailer. It's easier to heat and cool and quieter. Don't have the issues with trying to keep the windows and other openings sealed as you do in the corrugated aluminum siding. Holiday Rambler had a smooth exterior aluminum trailer. Fiberglass is a lot more forgiving in a hail storm as well which is a big deal in the Midwest.
Aluminum trailers are lighter, easier to replace damaged panels, less expensive to purchase. Aluminum sided trailers would act like a Faraday cage in a lightning strike.