The boom bag is a cover that protects your main sail and stays attached to the boom all the time. It is held in place using the lazy jacks and battens. Makes pulling up and dropping your main very easy and aesthetically looks good because it hold its own shape.
Any sail cover should be made from breathable canvas this will help prevent mold from growing on your sail, depending on what sort of sailing you do this cover can be made to roll up to the boom when sailing and secure out of sight. Construction of a stack pack needs to be a lot more extensive as it remains in place all the time in all weather conditions.