Lu Bu is wearing armour that actually covers his body. Additionally, the type of metal both were using are very different since, even though both warriors fought during the Iron Age, the metal Lu Bu has regarding his weapons and armour has more than a 600 year gap on Leonidas.
The metal of Leonidas' shield itself, around the timeframe of the Greco-Persian Wars, would have been bronze, his weapons would likely have been iron, and (even though the 300 version of spartans wore little to no armour) Linothorax armour would have been bronze as well.
As for Lu Bu, the most common type of armour (since he actually wears armour) during the Han Dynasty was lamellar armour, which could and Lu Bu's Ji halberd was also iron.
While Lu Bu has no shield, lamellar armour would save Lu Bu from a potentially fatal blow to a vital area if Leonidas were to hit him. Both of them have iron weapons, but the lack of armour outside of greaves for Leonidas is what screws him over in this fight. His shield will definitely be his saving grace, but it's his only protection against iron weapons and armour. Carbonized iron is a stronger material than bronze, and the fact that Lu Bu is wearing the more stronger metal for armour and protection gives him an edge in this fight. Both of them are undeniably skilled and likely comparable, but with Leonidas' Peak Human stamina in comparison to Lu Bu's High stamina means that Leonidas will tire faster than Lu Bu.
For reasons of metallurgy, armour protection and stamina, I give my vote to Lu Bu.
