Howdy,
Both have their uses, I like green stuff for smaller details or gap filling. Milliput I have not used as much, but I have used it to bulk things out or for larger areas.
Truth be told, I have used Sculpey for larger projects as a base to build off of, mostly because it is much chaper than either green stuff or milliput, and it is just a filler for details to be added on later.