I get the feeling that most successful Marth/Lucina mains are going to switch off between them depending on matchups.
If Lucina existed in Brawl, there are already a few matchups I can think of where I'd wanna use her mechanics instead of Marth's. High mobility characters like Sonic come to mind, for example. Since he is difficult to space against, the added damage of Lucina's non tippered attacks would be a huge bonus.
The other area where Lucina would have thrived over Marth is in edgeguarding characters with weaker recoveries. As Marth, edgeguarding an opponent with a tipped fair/bair would send them at an upward diagonal, thus making it relatively easy to still recover. A non tipped fair/bair however would send them at a much more convenient angle for gimping. Since Lucina can probably send her opponents at a more horizontal trajectory with her full length aerials- in theory, she should be more adept at gimping recoveries.
Against characters who aren't likely going to get gimped no matter what angle you send them at offstage (Brawl MK, for example), Marth would be more effective. Because the added knockback of a tipped aerial offstage is more likely to actually send the opponent out of the blastzone.
Just some food for thought. Currently I plan on using both depending on matchups. My intuition tells me that Marth will be more useful overall.
