There's sort of a herd-mind kind of thing on as far as who's better, and the opinion swings heavily in the favor of one character based on the level of play. Low-level players think that Lucina is better because she's more consistent, whereas a more mid-leverk player would argue that with enough skill a Marth should still be getting tippers consistently (I admittedly used to be a part of this group). While this is true to an extent, once you get to high level play it becomes more apparent that it isn't so much in the ways that really matter (i.e. securing kills with fsmash, the most dangerous punish tool on either character).
Ultimately, one isn't strictly better than the other. It depends on the matchup, and probably the player to some degree. It comes down to a pretty simple question- "can you get a tipper Fsmash on x character with some level of consistency?" if you're not confident enough in your ability to do that, Lucina is more likely than not the better pick. Feel free to correct me on that, though.