I believe Lucina has some advantages over marth when it comes to dealing with characters that are good at zone-breaking. Characters that pretty much can intrude your sphere of control easily like sheik, falcon, little mac... Marth usually relies on spacing, but once the opponent gets through there he becomes much less of a threat. With Lucina you won't benefit as much from zoning, and will usually chase the opponent instead, often taking the offensive.
If i were to think of both marth and Lucina as zoning charaters, as the general public seems to think, then to me, marth would be a defensive zoner and Lucina would be an offensive zoner. Don't really know how to explain this, but i guess it's just a different mindset.
I have tried Marth, but there was just something that didn't click with me. Landing tippers ain't really a problem, quite easy in fact, but i didn't feel much difference between playing the two.
So why do I play Lucina? it just feels more fitting, not having to rely on mechanics to seal the stock.