I do like both Mario and Luigi, but I like Mario better. Part of the reason is probably because overall, my favorite character in anything is always the main character. It's mostly their story after all; they're the ones who ultimately move the plot forward, the one we see the most of, and the character everyone depends on at the end. Unless of course that main character is trash, in which case I wouldn't be able to find much reason to like anyone else in a certain series.
Kinda like me, though I do have a very clear preference for Mario. I really like both, though, and Mario without Luigi is like spaghetti without sauce.