What should I use?

Q: May I ask what is a good sculpting material... Magic-Sculpt or Apoxy-Sculpt?

A: It all depends. (How's that for a definitive answer?)

If you intend to sculpt a model that will have a mold made locally (by you or someone close by), then I recommend KleanKlay or PaperClay. Both give you hours of malleability. KleanKlay will stay relatively soft and can be smoothed with a wet hand. PaperClay can be smoothed with a wet hand as you're sculpting, but it hardens overnight and can be smoothed further by sanding after a few days.

If you must ship your model to a mold maker, then I recommend you use PaperClay.

The downside of using Magic-Sculpt or Apoxy-Sculpt (or the new option Free Form Air) is that you only have about 20 minutes max of malleability. I wouldn't recommend it for a full-blown sculpture. The products are great for limited additive sculpture on an existing head and for securing mechanics. Too much and they add objectionable weight. Free Form Air is much lighter, but you still have limited time to work with it.