Both are fun, but Luigi just has a "lol" value about him that makes him satisfying when you land one of his ridiculous KO options (DTaunt, Sweetspot Up-B, Sweetspot DAir).
Mario is basically "space carefully, rack up damage, then maybe kill or gimp"
Luigi is "try to get in to the opponent's space and ****"
Mario is basically "space carefully, rack up damage, then maybe kill or gimp"
Luigi is "try to get in to the opponent's space and ****"