Dembele is an exceptional talent and has a higher ceiling but Martial has proven himself more than him. In his first season he scored 17 goals when at Dembele's age and looks to beat that this season with 11, whereas Dembele has never scored more than 10 & is yet to score at Barcelona. The second season Martial had to deal with losing his wife and a system change with a new manager who didn't trust his abilities. Martial at 20 definitely possessed more of a eye for goal than Dembele.
As a French fan I know them both very well. Martial is the better scorer (finisher), but Dembele is the better football player by a country mile. He's the one who has proven the most.
In France team, Dembele already showed a lot more than Martial in less time. Martial is a sub at best in France team, he has a hard time even being on the bench right now. Dembele would probably start if he was fit, that's how good he is (watch games like France - England, Dortmund - Bayern cup final and tell me Martial is better). And you mustn't watch a lot of Bundesliga, because Dembele was one of the best players there last year. In his first year there and at age 19.
He just couldn't show that in Spain yet but that's only because he's injured. Dembele played like 1 game and a half for your team ffs, if you want to know him watch Dortmund / France videos, but stop making comments about an injured player who hardly started playing for your team.
Or do you think clubs spend 150 million on an average player ? Only a few players in the world are worth that sum (less than 10, even today)