Luigi. F Yeah.
Except for the old DIC cartoons and the live-action movie (which I'm one of ten people that actually LIKED IT), Mario doesn't have a lot of personality going for him. Kinda like Link, I guess Miyamoto intends for him to be the link between you (the player) and the fantasy world you interact with.
Mario is the cookie-cutter hero. Everyone knows he'll do any heroic deed, so they always go to him. Not to say I hate the guy, just that he's quite one-dimensional when it comes to the games.
Luigi, always reserved for 2P games or side-character roles, has actually gotten some character development in newer games. Interestingly enough, a trait of his from the DIC cartoons (that he's somewhat cowardly) carried on to Luigi of the video games, first seen in Luigi's Mansion (which I also liked; deserves more credit than it gets). He's scared witless in the game, however he still goes through the ghost mansion to save his bro (hey, he's family!). He may be a coward, but he has guts.
Another interesting trait about Luigi that's developed in the games is that he isn't well-known in the Mario world. Despite having always been his brother's sidekick (and rescuer when Mario was MIA), seems like few know or respect him. In Mario & Luigi: Superstar Saga, I recall someone refering to him as a Mario fanboy who got the colors wrong. Others had trouble remembering his name or questioned his reliability. I found this pretty amusing.
It's kinda amusing, but it's thanks to America that Luigi first got his unique set of sprites. Nintendo of America had deemed the Famicom Disk System game Super Mario Bros. 2 as too hard (and it is; I've beaten it, but it's more of an endurance test than a fun video game DX), so another game, Yume Koujou: Doki Doki Panic, was modified into our SMB2 (which was later brought back to Japan as Super Mario USA). This was the first time Luigi wasn't a simple recolor of Mario in-game. Even SMB3 and SMW didn't originally follow this trend, not until their remakes (All-Stars, All-Stars+World, and the Mario Advances).
(as an interesting side-note... Shigeru Miyamoto had more to do with the development of Doki Doki Panic than SMB2J!)
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tl:dr version: Luigi is awesome.
both for me
im partial to Mario cuz...well Mario is my name XD
and i like luigi cuz hes awesome!!
Xairmo = anagram for "Mario X"?