As I understand it, the Booster covers the material that is not covered in the PrepCast, so you would need to do both to give you full coverage of topics for the exam. I'm sure someone more knowledgeable than me will confirm, but I know that's what the product was designed to do. The Booster comes with the checklist, so perhaps you could use that as a starting point to make sure you've got all the topics covered and are only reviewing them once?