How much attack do you need in order to use Draconic Aura instead of Luna?

You shouldn't just think of it in terms of attack power of the unit you are putting the special on.

Take an example. Draconic aura adds 30% attack power, so on a 50 attack unit, thats 15 attack power.
Luna has to hit a 30 defense unit in order to match that. When your enemy has more defense, Luna becomes better. When it's lower, Draconic Aura wins.

So when you look at raw values, there is a 'breaking point'. But again, don't look at it that way. Because when the target has less defense, you will be able to kill it anyway, you may not even need a special proc to do so. So Luna has added value when a target becomes to beefy to kill, by adding damage off of that target instead of your attack power.

Also, DA scales poorly. 15 damage from 50 attack sounds good, but it only increases to 18 damage when you go up all the way to 60 attack, which is not easy to reach in the first place.

I generally abide by a rule of 15, running a fixed-damage special if the unit can hit a guaranteed 15 dmg or more (50 atk with Draconic Aura, 30+ def/res with Bonfire/Iceberg).

Besides that, I prefer running defensive stat-based specials when possible, as the damage scales better than with Draconic Aura (as Cygnus stated).

Luna is largely better for things like lower dmg quad units, to break through high defense units. That's not even touching on which builds to go for lower or higher CD specials.

IIRC Elincia runs Luna about as effectively as Draconic Aura, so go with whichever you prefer. And anyway, like Cherche, Elincia rarely needs her special anyway to nuke most units regardless of color, so she's always a good candidate for Galeforce or Aether for arena scoring. Mine is +spd/-res as well, and I'll equip Galeforce the first chance I get after using her on a flier team and seeing how stupidly hard she hits.