From what I understand, your skin gets 'used' to salicylic acid, and becomes 'resistant' to it after a while, whereas that doesn't happen with benzoyl peroxide. Most 'over the counter' acne products have salicylic acid. Proactive has no salicylic acid, but both the repairing lotion and (unusually) the face wash has benzoyl peroxide.
Having used both on my stubborn moderate acne, I can honestly say neither has been very helpful to me, but other people swear by it, so I do hope it works for you.