I'm sure this thread has occured in the past, but I'll answer anyway. I like both of them, but I prefer Luigi because he looks more like me (I'm far skinnier than I should be), his personality is more similar (I'm somewhat of a coward*), and he jumps higher. Hmm... I suppose I like Luigi more than I thought.
*See my response to the SM64 piano in "The Horror... The Horror!"
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"Chocolate milk as a world-wide delicacy! Who agrees? Anyone?"
- Ian "Suffix"