For sculpting it is up to you as to what you prefer, if you like a softer clay, buy a softer grade of Chavant.
Most of the time I use a Medium grade Clay, not too soft and not too hard.
You can also use a Roma Plastilina NSP (which stands for Non-Sulphur Plastilina) it's very similar to Chavant.
For most moldmaking purposes I spray on a thin layer of Crystal Clear Acrylic Spray as a release and then do a Matrix Mold with a Fiberglass Shell and a silicone interior.