Martial is special, apart from Dembele, Gabriel Jesus (?) Emre Mor at a stretch, i can't think of many attacking talents under the age of 21 near his level, he has the ability to coupled with the athleticism and technique to be a b'allon d'or winner something i don't think Alli despite his huge talent has it in him to do.
I think the best part of his game will always be his numbers, which isn't a bad thing to have going for you, but in the end, when someone like Martial decides to 'switch it on' his scope and upside is bigger than Allis because he is simply a better player in most phases and aspects of the game attacking wise.
Alli is a unique talent as well, but in a modern comparison it would be like comparing a Thomas Muller to an Eden Hazard, one is a great player who got to near the top of their game based on intelligence movement and cool finishing, while the other because of his mercurial maverick talent, one has it in them to be the best in the world, and the other despite getting the very best out of their talents simply doesn't, Alli is probably even more special than Muller because he is genuinely a central midfielder by trade, while Muller has been a forward throughout his career, but still there is a ceiling in which i don't see with Martial potentially.