Wallace was grounded in the real world. King Leonidas from 300 was not. As characters, William Wallace is no different from one of us that could do a good job wielding a sword. King Leonidas is apart of a warrior civilization. The Spartans were the greatest and most tactical warriors, and with 300 men, defeated thousands. Like MMJRK said, Wallace and his army of hundreds took out hundreds of soldiers. Leonidas with his 299 fellow Spartans took on thousands of the Persian empire, which had many different varieties of armies and fighting styles, let alone not being grounded in the real world.
...that's just me.