Marth is harder than Luigi, this is fact. If he spaces you, you are hardpressed to do anything. His fair > all your aerials including dair. You can do NOTHING to him offstage if he's competent. This is another character where most of the damage you do is from the ground... which sucks when he has as big a range (relative to Wario) as he does. Marth also has very good ground speed, so if you're not careful he can easily sneak a grab on you, and that's a free grab release > upsmash for an early kill right there.