They are totally different medications. Salicylic acid is used to shed the top layer of skin cells faster, preventing them from clogging pores. Benzoyl peroxide is used to kill the bacteria that causes acne. That is why it would be best to use both, but don't overdo it all at the beginning, or you'll have other problems like dryness and peeling. Hope this helps, I had problems for years and am pre-med so I've had a LOT of experience with all this acne stuff :)