I've been watching a lot of base building videos for mini figures, like Warhammer. Most of the builders use DAS air dry clay. I bought some and am going to use it on a pony for a contest, just to see how it holds up. Apparently, it's the "professional" air dry clay and it's supposed to be pretty sturdy. It holds texture incredibly well and it easy enough to work with. You can smooth it with water like regular clay.
I will post my experience with it once finished.