I use the body filler as a quick reinforcer and something that's sculptable for detail. The resin in addition to the fiberglass cloth gives the entire piece strength. A couple resin coats on the outside also will make it tougher and more badass as well as let you get rid of some of those stark fold lines. But make sure you reinforce it first. And there will be a fair amount of sanding involved.
But trying to decide between them, I can't.