So while at SF, I got many chances to play with many people, I made friends and practice partners with tons of Marth players, and I used that chance to sneak in and practice my Lucina. And I improved a lot, even started beating local players that I was unable to before.
While at it, I noticed that both characters' advantage states are very different, Lucy's idea is to ram in and using her mobility to fake out and force reactions, while Marth positions himself where the opponent might be and places his sword with high precision, while non-tipper moves are capable to make combos neither Lucina or his tipper moves would.
I don't think neither is inherently better, maybe Marth's is because if he tries Lucina's way, a stray tipper can change the result of a match.
But anyway.
During my way back to town, I was wondering "how would I improve Lucina with patches/edits? (obviously without anything too crazy)".
It struck me that having a bit more KBG on some key aerial moves (Uair, Bair, Nair) would make wonders for her, since now her juggle game would be truly fearsome, as they would actually kill, would put opponents on bad states, and throw them offstage easier, maybe even gimping better. At least, that was the idea I had until I went and check both characters' frame data, since I was under the wrong idea that Marth's aerials only dealt more damage while having the same BKB and KBG, HOWEVER, Marth's tipper aerials have better damage output AND better knockback values.
But it doesn't end there, Dancing Blade is the one where the tipper only has better damage, but the difference is very noticeable, it is actually able to kill without all the rage Lucina's would need, all while not being too inferior damage-wise than the granddaughter's.
This whole read killed my hopes for a high-level Lucina, she would never be able to cheese victories like Marth can, and everything she does, a tipper might make Marth do better. Even my proposed chance is something he is already capable to do.
I still like Lucina more, and feel much more comfortable with her, but results and frame data all point to Marth being by far the superior choice.