I'm not much of a sculptor, but a good friend of mine is and has done some sculpting for GW's Inquisitor line, and I know a lot about molding and casting. Green stuff, or any two-part epoxy putty, has been used because it will survive the vulcanization process for making molds for spin-casting. (Which GW no longer uses now that they have FineCast.)
That is why they use stuff that is sticky, tacky and hard to work with, but if you are sculpting for your own use, or to cast using RTV silicon or other low-pressure, low heat methods then Sculpy will work just fine. In fact, it might be preferable since it will remain pliable until you find the shape you want, then you can fire it to harden. I think it can be brittle when dry, so Green Stuff will be more durable in the long run. Green stuff is also better for conversions since it will stick to the model your converting, but for a scratch built sculpt it will matter less. -Mark