I would have to vote for aluminum. The factory built FRP units have very little innerstructure (mostly a few 2"X 2" wood sticks) glued to the back side of the FRP. This ain't much to hang cabinets from, nor much when (if) you would happen to be involved in an accident. Then there is the problem of repairing the stuff. Any kind of major damage would require a complete panel replacement. There isn't any way to splice FRP that I know of.
Most aluminum sided units have a welded steel square tubing structure. This basically forms a roll cage. Damage can be repaired a section at a time by removing the affected alum panel & cutting out the bent sq tubing, then replacing that section only. Anyhow, thats my 2 cents worth.