Swift Sparrow or L&D?

Swift sparrow is the better choice when the character has enougth phisical bulk to take a hit, that applies also to mages, as if they become too frail having 1 more spd will be completely useless as the get oneshotted before getting into desperation range,
you can say that Ardent sacrifice or Reciprocal aid can be a solution for that but keep in mind that it will be useless after being used once, you can't run skills like Reposition or Draw back and your character won't be able to do much untill the 3 turn.
L&D is particularly usefull on character that have high bulk and middling spd, in those cases the loss on spd is offset by the fact that they get doubled much less, naturally a Breath set is always the best choice for those character L&D is the best choice after that in my opinion.
Sometimes Fury is the better choice compared to Swift sparrow or L&D because the chracter start with too low of a bulk, or in Celica's case where Fury is the best choice as A slot to get her weapon effect at the max potential as soon as possible