I think Nico would win, because he has power over the dead. He could summon the dead souls to come and take Percy away. Leo control , but Nico knows how to control his power. Nico can also shadow travel, so he could avoid any attack from Leo. Leo is also hotheaded, so he doesn't make his decisions thoroughly
I believe Leo would win. Though he is younger than Nico, he has more physical strength. As well as this, he is also a tactician and has literal fire power. He is inventive and good with 'on the spot' thinking and has more allies. Yes , though a low blow. More people trust Leo than Nico, not many, but some none of the less.
Nico has more power, though. He is a child of one of the Big Three, Hades' child. He is the darkness that swallows fire, and he could easily extinguish Leo's flame. Besides, Nico has a wicked sword, and who knows what strategy, intelligence, and warriorship lies behind his veil of mystery?
Leo has more self control with his emotions and his powers. Though he acts childish, he is a true genius. Given a proper motivation, he could defeat Zeus. He has an entire ship full of weapons, so Nico's demon sword is practically null and void.
Leo could incinerate them just by looking at them. Doesnt matter how strong a soul if their is no body to control and attack with. Also, Leo has his belt of tools, he can pull out any supplies neccesary.
O.o well, I kinda doubt that leo could actually incinerate them by "Looking at them" because, well he's not a basilisk or anything.... and Nico COULD call on his dad if need be....besides, though he can only summon so many, it would depend on what he summons.... like hellhounds or something...