Dark Breath

Both breaths have their niche, so really which one is the better breath depends on the Dragon you're using and what role they have
Refined Lightning breath comes with a built in distant counter and a neat new effect that calculates the damage on any ranged units using the foe's lower defenses
Refined Dark breath comes with -7 atk/spd on both the target and any enemies within a 2 space range, and even has the new effect that lightning breath has, but you'll have to change your A slot into distant counter

I use Dark breath and work very well but i think light breath wwors better if you want tank, because dark breath only debuff a single target many times ligth breath is a good support if you put DC allow to all your team reach insane stats even more if you use a Blade tome unit