How is Leonidas more experienced? The movies suggest just the opposite.
Helms deep is easily comparable to Thermopylae. In fact, the battle at Helms deep was inspired by Thermopylae. This is why, originally, there were only something like, 300 men at Helms deep. This is neither here nor there though, Helms Deep lasted only a night but it was Aragorns presence that gave the men the morale to fight. I only bring this up because it shows that Aragorn, like Leonidas brings morale to the men around him.
We see Aragorn fighting Orcs and other inhuman beasts that dwarf Humans in size and strength. Aragorn cut through Hordes of enemies (alone) with great ease and with no shield. He is a masterful swordsman.
Leonidas is a badass for sure but his strength comes primarily from the men around him. This is how Spartans fight. We never really saw Leonidas fight without the aid of his men. I'm not trying to suggest that he can't but the Spartans fighting strategy relies on the men around them. We have, however, seen Aragorn take down small groups of Orcs, alone.
Aragorn is the better swordsman and from what the movies have shown us, he is much more battle hardened. The only problem i see Aragorn having is Leonidas' shield. A spartans shield is an extention of their body. The shield could prove to be a problem.