Sculpey III is less brittle than Sculpey, but still not really suitable for jewellery, although if the jewellery is looked after, you may get away with it.
Sculpey III also bakes with a slightly matt finish.
You are better off using Fimo Soft. Fimo classic is an excellent strong clay, but difficult to condition.
Kato clay is also excellent, but again, difficult to condition.
My personal preference is Premo. It's easy to work with, bakes hard with a slight flexibility, has a terrific colour palette, mixes well and is just the best.

Also, it's no dearer than the others.
hth, hun.