Neither, although I think Leao's potential is a step above Nico's.
Nico would demand a huge salary and we can't spend that kind of money on a guy who looks like nothing more than a good backup. Profile-wise he's also a very passive ball-to-feet player, not a space-eater, which is the biggest quality our LW needs.
Leao also not ideal profile wise, doesn't like to press much, slightly better off ball threat than Nico but still not a space eater, lacks consistency in his game though, sometimes looks unplayable then plays like Abde the very next game, tragic.
If forced to choose between the two, I would go for Leao, assuming Milan are willing to take about 60M. But I think a striker is the absolute top priority for the team, our potential is limited at the moment due to Lewa being physically washed.