Lucina's color swaps >>> Marths
On the serious tip though (ha), even if a 'good' player will on average deal more damage and knockback with Marth and have more kill options than Lucina, he just doesn't feel as good to me to use. Maybe it's that his animations seem less fluid because of the size difference, or the slight delay in making a jump compared to Lucina (shes shorter but with the same jumping abilities so she appears to leave the ground, including her hitbox, faster than Marth), maybe it's that I rely on sliding upsmash a lot and it's better on Lucina, maybe it's that you never feel bad on Lucina for hitting your opponent since it always does consistent damage no matter where in the hitbox you connect with, maybe it's that she seems to travel farther with midair shield breakers, etc. etc.
Lot's of little things just make it feel better for me to play Lucina, even though I know how satisfying it is to land tippers on Marth and get kills out of sideb up combo etc. in a way that Lucina never does.
They feel different, and I prefer Lucina's overall feel (and that's ignoring that I find her thematically more interesting than Marth). I suppose a good analogy for difference in feel for League of Legends players would be that Lucina is an all ad + Last Whisper Fiora and Marth is Infinity Edge + Ghostblade Fiora. It's not a perfect analogy because crits are psuedo random and not 'tippers' but it's pretty close.
As it stands right now, Marth is a slightly better choice from a numbers standpoint, but he feels bad to me and that's not something that some extra hit stun and another kill move or two will fix. Besides, this game is in it's early stages and anything could happen. Next month someone could invent a super rushdown movement technique that doesn't work on these two characters which would make it all but impossible to tipper and then suddenly Lucina would be the objectively better pick. Or maybe there will be a patch that takes advantage of the gathered online data and all the 'clones' will get slight adjustments to their numbers to make them not just 'better'/'worse' versions of the originals.
Either way, I'm gonna be using Lucina.