I think Leao is very overrated but the difference between him and Nico is night and day, Leao is faster, stronger, a more versatile dribbler and can create his own chances too.
The fact that he's taller + has a bigger frame makes him almost impossible to stop once he gets going, just watch the second half he played at Bernabeu a few weeks ago, he was blazing past Madrid bodies like no big deal, even guys like Militao who are physical freaks COULDN'T compete. Heck, you can even watch the Portugal vs France Euro game where Kounde looked like a grandma trying to stop him.
Nico on the other hand rarely imposes himself against opposition fullbacks unless they're super washed and his dribbling bag is also limited, he rarely goes for the weak foot cutback which makes him predictable.