They are not competitors and they are not interchangeable. A BHA exfoliant is an acid that clears the pore and lifts dead surface — it is a step you can overdo, and doing it nightly is the usual reason skin becomes sensitised. A repair serum is a leave-on treatment meant for every night. If you can only run one, run the serum nightly and add the acid two or three times a week; that order is the whole answer and it is the reverse of how most people buy them.
The order that stops the acid from backfiring
Acid on the nights you use it, serum on all of them. Skin that has been exfoliated too often reads as sensitive, tight and reactive, and the usual response is to add more products to calm it — which is the loop worth avoiding rather than treating.
Neither is a moisturiser
Both are steps that go under one. If skin feels tight after either, that is the missing cream talking, not a sign the product is wrong.