I really think the main difference between the two is that Mbappe knows his game better than Dembele knows his, or rather he knows his limitations a bit better than Dembele who acts like he doesn't have any limitations, as if he can make any pass, dribble any player, which will result in silly ways of losing the ball (and sometimes in extraordinary goals/assists). Mbappe explores his game as well, but with a much clearer understanding of what he can and cannot/shouldn't pull off, at least that's the impression I get from him. Obviously Mbappe is the clear favorite to come out on top of this generation, but I still believe Dembele has a great chance to become the main guy of this up-coming generation. If I had to make a safe bet, I'd go with Mbappe, but with good odds I'd take a swing on Dembele becoming the best, he just needs to want it as much as Mbappe does but Mbappe is a bit psychotic in that sense so it'll be difficult to do.