Lucina or Mia?

You should should go with Mia since she is better overall. Unless the Lucina in question is actually a Brave variant, then use both. You can patch Mia's Speed with Speed+3 seal.

Mia is a lot better, even with a -SPE nature. Lucina is more of a niche unit that's best off as a combat medic in Tempest Trials and the like.

Like TheSeunger said, give her a Spe +3 seal since it helps her killing potential; a quick simulation with her base kit shows she gets 36 more kills with it.