300 spartans demolish; maximus is arguably the better commander but Leonidas has a better army from top to bottom, each Spartan is a wrecker the same can't be said about Maximus' platoon
Maximus has better armour and skill arguably but Leonidas has better physical feats. Id go with Maximus based purely on his armour tbh; his skill is enough to be on par with Leo but his armour (lack thereof) would be the deciding factor
Battle 1: Spartans roll on over the Romans. As relentless1 said, all the Spartans are some level of Zack Snyder Watchmen super soldier type, which isn't the case for the fellas Maxi is going to have.
Battle 2: Again agree with relentless, Maximus has far more 1v1/dueling fights with varying circumstances in varying scenarios. Still will be a tough fight, and if Leo can catch him with a slow-motion spear throw, he can win. However, I'll give Maximus the dubya here.
I don't think Maximus could take Leonidas in a straight fight. Sure his armor is better. But physicality-wise, as it's been pointed out. The Spartans are superhuman. Slightly. Definitely low level. But strong/skilled/fast enough to make regular people look like cannon fodder. He'd blitz through Maximus.