Both. Mario is the original, has a good look, and is the hero in nearly all of his own games and spin-offs. But in some games, you can unlock Luigi. He's faster, taller, and jumps higher than his brother. But he stops slower and is scared of a few enemies. They've both got their ups and downs.
People say nintendo doesn't use him, but that might be because they want Mario to be the better one. They gave him his own game, which I liked very much, and in the Galaxy 1, he's almost like like the secret unlockable character that you always want to play with, but have to beat the game first. I pick Mario, the original.
