I've been doing some sculpting and casting with Milliput recently. I find that it's very easy to mix and work with. It sets quickly and the result is very hard with a lot of detail possible.
The downside?

It's extremely brittle. Quite easy to snap in your hands and it's liable to shatter if dropped on tile. Thankfully the breaks are always clean so it's easy to glue the stuff back together but I'd like something less brittle.
I've read that some people have had luck mixing Green Stuff with Milliput and that the result is still on the firmer side like with Milliput, but less brittle like with Green Stuff.
Have you tried mixing Green Stuff and Milliput? What ratio do you use?