Here's my opinion as someone who played Marth in tournaments for over 7 years, placed well in nationwide tournaments (even if our scene is weaker) and has the game right now.
They both are going to play very similarly, they have incredible air control and ground movement, you "weave" around your opponent with sweeping hitboxes while returning to safety, after all they are very similar. Rushing your opponent down straight up won't work as Lucina and there's better characters to do that. However, they have distinct strengths, initially I thought Lucina would be just a weaker version of Marth because you control your game around the tip of your sword. It's true that Marth has a better combo game at low %s if you hit tipper aerials, it's hard to put this in words, but hitstun in this game is still very small and Fair has considerable landing lag now, and combos into Fsmash don't seem as good as before because its range got nerfed so much. Combos feel situational, for the most part you'll be forced to carefully go for "one hit at time while putting your opponent in disadvantageous situations". Tipper Fsmashes are crazy yeah, but at the same time they are very inconsistent to hit, and Marth still struggles with his old problems of killing at high %s. This is where Lucina has over Marth, a fresh Fsmash kills around 110% no matter where it hits, going offstage to edgeguard with fair is much better with her because her fair sends your forward almost in a straight line, while a Fair tipper isn't likely to kill your opponent offstage because it'll send your opponent upwards. I had better success with Lucina, if just for easier kill moves, considering the blast zones are so big it's mind boggling. Or maybe I'm just bad at adjusting to the new tipper range.
agh this is so terribly written but i hope i got my point across