Benzoyl peroxide and salicylic acid are both excellent acne fighters, and definitely the most affordable treatments on the market. Almost any drugstore acne treatment is going to have one or the other. But it's impossible to tell from the bottle which product is going to help clear up your specific acne. If you end up with the wrong one, it can actually make things worse. To help you determine which product you actually need, I've put together a quick comparison. Ready to know which one you need once and for all? Then just keep reading.
When you use benzoyl peroxide, what you're actually getting is benzoic acid and oxygen, because the product breaks down on contact with your skin. The oxygen helps kill P. acnes, the bacteria that contributes to acne. Benzoic acid causes skin to peel and dry, exfoliating out the plugs of dead skin and sebum that inflame follicles, causing bumps. Benzoyl is sold in concentrations up to 10 percent, but studies show that concentrations of 2.5 percent are just as effective and much less irritating than stronger ones, so that's what I'd recommend you use.
Benzoyl is a powerful product that can penetrate pores deeply, but it can also be very drying. If the spots you're seeing are deep under the skin, red, and/or pus-filled, benzoyl is probably a safe bet because you need both bacteria killing and deep-down exfoliation. Neutrogena On-the-Spot  ($7) is one of the most popular benzoyl treatments, and the Proactiv system is also benzoyl-based. If you've got sensitive skin and light acne, though, think twice before you pick this one up because you can actually exacerbate the problems you're having if you over-apply.
For the sensitive type and people plagued by blackheads, salicylic can be a life saver. Salicylic acid, like benzoic acid, is a good peeling agent, though it's gentler and lacks the oxygen to help kill acne bacteria.
If you have lots of blackheads and spots but no deep, closed comedones , salicylic is the best choice for you. Clean & Clear's treatment  ($8) is mostly salicylic acid-based, as are the Oxy pads  ($7) you see everywhere. Salicylic won't be as sensitizing and is less likely to dry you out. Again, don't overuse it — doing too much actually makes things much worse.