That depends. Most people would do best by choosing the Cast Drawing in Sight-Size self-directed course. It gives you the most content at the most affordable price.
Both will teach you all you need to know about cast drawing in Sight-Size. The differences are that Cast Drawing in Sight-Size: An Atelier Manual is missing most of the history and theory of Sight-Size. That content is in The Sight-Size Cast. Also included in The Sight-Size Cast is how to paint casts in Sight-Size.