Tails has his own personality. Luigi is like a second Mario.
He is Mario's younger twin brother.Luigi IS Mario's younger brother.
Luigi. Because he is Mario's younger twin brother.
Tails. He's a 6 year old fox that can fly by using his tails. Tell me that is not awesome. He may be annoying in some games, but what about the games where he isn't? He's pretty awesome. And he's also my favorite Sonic character, while Bowser/Yoshi are my favorite Mario characters...so it's no surprise that I chose him over Luigi, really.
Luigi = Mario Recolor
Therefore, tails is better.