I also looked at used Sidekick trailers, some were FRP and others aluminum. As far as weight the FRP is heavier as it is fiberglass reinforced plywood. One downside to that is if it has any delamination it is a point for water to enter and thus rot the plywood. I would look it over with a fine tooth comb. Not to say they are bad just something to be aware of. The aluminum skinned trailer will obviously be lighter in weight, and ultimately lower maintenance over time. Check the aluminum floor under the mats for corrosion as this can be an area that is neglected. The ammonia from urine will react with the aluminum over time and cause pitting. I went with the all aluminum and am EXTREMELY happy with the Sidekick brand. It is a high quality trailer that when they were in business was one of the top manufacturers of horse trailers. One downside to them no longer being produced is parts being hard to find should that need arise. I hope this helps in your decision process. Happy Trails!!