The difference that decides it is chemical rather than a matter of strength. Salicylic is a BHA and oil-soluble; glycolic is an AHA and water-soluble. That is why the two are sold for different complaints — salicylic ranges are marketed at blocked pores, blackheads and skin that breaks out, glycolic ranges at dullness, rough texture and flaking — and why buying on the higher number gets people the wrong bottle. Match the acid to the complaint on the label, not to the percentage. Both are acids and both can irritate: the maker's directions and a patch test decide how you use it, not this page.
Not at the same time, and not every day
The instinct on owning both is to use both, and both cartons tell you not to. Pick the one whose label names your complaint and build up from the frequency printed on it rather than starting where you hope to end. If the skin objects, that is the instruction to follow. Nobody here can prescribe a schedule for your face; the maker's directions can, and a dermatologist can better.
The percentages are not comparable
A 2% salicylic and a 7% glycolic are not two and seven of the same thing. They are different molecules doing different jobs, and the numbers do not translate between them. Within one acid the percentage matters; across two it means nothing, which is why the useful question is which acid rather than how much of it.
