Never heard of Apoxie myself, but I've found anything epoxy like tends to set fast, so working time's a possible problem.
As far as Sculpey, depending on what you want to do with a custom (IE: Don't care how fragile it is) or what type you may be using, these are the factors that can vary results. The super sculpey and primo are supposedly good. I can't even find them.
Sculpey III though, which is all I can ever find, is more like a moderate modeling clay and a good "kids" clay. I have very bad luck with it for anything really, but I can't find good Sculpey anywhere. Just III.