You fools, can you not see that the great Yoshi rules over them all? Oh wait, only between those two brothers. Go read my comic, (un)official proof that Luigi is stronger (he can cast Ultima ^.^), Luigi can be in a random kitchen then ahead of Mario in Starlight Zone so he's faster, and Luigi is evil, how can he get cooler?
On non comic terms, Luigi is faster, I think he's cooler, and he kicks Mario's derierre in SSBM. Luigi wins. Then he gets eaten by Kirby, who gets eaten by Yoshi, who gets stabbed by a Black Mage, who gets run over by Metal Sonic, who gets his butt whupped by Shadow the Hedgehog. Shadow wins. w00t