I'd say, IME
Brake cables & outers, not much difference from one to the other in terms of performance or longevity.
Gear cables, it is definitely worth buying a good set of outers as this will have the biggest impact on gear indexing, the inner, not sure how much of an effect it has tbh. I have never run a cheap cable with a good outer to find out, if I bought a good outer, I'd buy a good inner to go with it.