Remember that fiberglass doesn't conduct electricity, so u have to run a ground wire to the chassis for all ur electrical components.
Aluminum tubs are nice, light weight, and also don't rust. However, they are pricey. And unless ur good at tig welding aluminum, everything u add to ur tub will have to be bolted or riveted. I have never seen an aluminum tub in person, but I would question the manufacturer about how they fasten threaded inserts (like where u bolt the fenders, dash, and seats) to the tub. I'd prefer slugs welded into place with steel timecerts (? Much stronger and more permanent than helicoil) installed as opposed to a steel nut caged in place. Those will drive u nuts (no pun intended) if they ever break loose.
I'm sure u know all there is to know about steel tubs, not much to explain there.
Good luck with ur purchase, keep us posted on what u decide on.