We're asking/discussing these questions constantly throughout multiple topics, and all it is speculation/guesses.
Let's keep a thread like this out of sight until we do have some things to base stuff off of (tournament wise/etc).
I will say that at a base level, both Marth and Lucina are spacing characters, and Marth's reward for spacing well is just better than Lucina's. Tipper smashes kill a lot earlier in this game in comparison to Lucina's smashes, like 40% difference. [Aggressive] zoning characters always play with this notion of "hitting them X more times than they hit me" because without a camp/projectile game, getting hit is pretty common; this notion tells us that Marth is going to be able to kill like 4-5 hits earlier than Lucina will, but will be harder to land that kill move.
Otherwise knockback/etc differences could lead to one character having easier bread and butter strings/combos, this could favour Lucina, but could also favour Marth with untippered hits having less knockback (although not tippering aerials or tilts is usually suboptimal for marth in other games, so...)
