The two main advantages of Structolite for scenery making IMO are its slow setting time (hours), and its perlite texture that simulates rock very well. You can mix a big batch and work all evening without having it set up in the mixing bowl.
I've also used the slow-setting drywall compound (45 minute variety) and added my own texturizer. It just isn't as strong as Structolite when set.
Gypsolite is another material almost identical to Structolite and available at many hobby shops in small quantities.
Jim
