I had aluminum siding on my 2002 Sunline trailer that we just traded last summer. It help up beautifully and cleaned up well, even in it's old age. The new Minnie has gel coat fiberglass and I find it much easier to clean. Now, the siding on the Sunline had a special coating and never needed waxing. The fiberglass gel coat should be waxed. Waxing was pretty easy using McGuires which made applying and removing pretty easy, and enjoyable with a couple of cold drink breaks!!
I don't know whether either would be a deal breaker for me. If I had found a really nice trailer that I really liked in terms of floor plan and construction quality, I would have purchased it with aluminum siding or smoothe side material.
