This is oversimplification pal...
It depends how the team plays and what exactly the RW profile is.
if the RW is supposed to be a wide forward, tasked with starting from the half-space and receiving in goal-scoring positions to finish, you go with Salah
if the RW plays on the touchline, tasked with creating space via dribbling and assisting in the box, you go with Dembele
That's the thing though, if those are the options to choose from, Salah is to be prioritized.
Much rather have a Salah in his perfect element than Dembele since Salah is much more of a game changer in reality.
Dembele is like a much better type of Adama in the sense that there's a lot of POTENTIAL to be a game changer, but he doesn't fullfil that role in a top top team. Dembele is still waiting for his exploding moment (for example like that CL season Vini had). He's 26 now but in a way it's felt like he's always been 22, maybe because of his injuries.
Salah would've done much better than Dembele at RW for Barca the last few years even if he wouldn't have a tailor-made role.