As for which one is better for your skin? Well, it depends. Both are forms of alpha-hydroxy acids (AHAs) but can have drastically different effects. "Glycolic acid is a smaller molecule than lactic acid, meaning it can penetrate deeper into the skin," says Riley. "That can make it more aggressive than its gentler sister, lactic acid, but [glycolic acid] also been studied repeatedly for its beneficial properties for acne-prone skin." Additionally, the glycolic version of Good Genes has a slightly higher concentration of acid in it than the lactic version, so that's something to take note of if you've got sensitive skin. (As someone with oily, acne-prone skin that gets easily irritated, I generally prefer the lactic acid formulation. That said, the glycolic one has a very similar creamy texture, but with a more noticeable tingling sensation.)