Cecilia Building

Even with a +spd and Hone Cavalry, she's not fast enough to be doubling much(only 35 speed without her having a speed boosting seal or A skill). She'll really only be doubling with a Breaker Skill or Brash Assault. I'd have to recommend Atk. You want her to have as much punch as possible for that Nuking potential. I might also recommend Death Blow and a +atk seal. Can add an extra 9 damage(not factored into the Blade Tome itself like Hone/Fortify Cavalry)

I personally use a +Spd -HP, she works wonders as with Life and Death, +Spd and a Hone Cavalry boost she can hit 40 speed; anything she can double gets doubled, anything that can't get doubled will likely fall to a boosted Gronnblade+ anyway.

That said, +Atk is cheaper with a Death Blow build, and will get similar results, personally I'm just a sucker for doubles and speed.

Cecilia favors +Atk the most. With both Hone/Fortify Cavalry buffs, she can reach amazing nuking potential. Even outright 1HKO'ing certain Red units.

Likewise, Death Blow is her primary A Slot skill. Attack+3 seal will push her nuking power even further.