Well, Themistocles coming into his career as a politician was popular among the lower class of athens. He became Archon in 498 BC (after Marathon). He improved the Athenian naval forces with an added 200 triremes for the up and coming battle at Salamis and Artemisium. He commanded the battles which was a huge success not jusrt for the Athenians but for all greece.
This battle was the the second blow to the invading persian army.
Leonidas, well his battle with no doubt firstly struck fear in to the Persians and sure was a speed bump in the persian succession.
Leonidas packed a real punch to the persian forces and Themistocles truly disabled the persian forces for the next and final battle with them at platea. (which we won)
Hope this helps