Hans,

I sell metals for a living. Without getting too technical, here's the difference.
Bar stock is rolled from a billet under heat and pressure. This compresses the molecules, and ensures a dense, void free composition, wit an even, unidirectional grain structure.

There are different methods of casting, but the basic method is to pour molten aluminum into a mold and let it harden. Die casting is a little better. There the molten aluminum goe into a mold similar to a plastic injection mold, and a more accurate denser part is produced, but it is still inferior to bar stock.