Obviously it depends on what you want to sculpt, but for years I've used what seems like a strange mix - Apoxie Sculpt and Super Sculpy, mixed together. 2 parts Apoxie Sculpt to 1 part Super Sculpy. This mix is a little firmer than Apoxie Sculpt by itself, can be easily smoothed with water and has virtually no shrinkage when hardened. Just make sure you mix them thoroughly.
Now, WHY, you might ask, would someone mix the two together? Because adding the Super Sculpy to the Apoxie Sculpt increases the work time quite a bit. Apoxie Sculpt is workable for about an hour - maybe a little longer before it hardens too much to sculpt. The Apoxie Sculpt - Super Sculpy mix stays workable for as long as three hours. And, yes, it is rock hard when hardened. I'd imagine that it isn't quite as hard when cured as Apoxie Sculpt alone, but it is quite hard enough for any/everything I've ever needed it for, and it sands glass smooth!