Finding out that reduce speed != slow makes me want to kill whoever programed this game.

>>536533708
It depends on what you can get more easily.
Her best build is green by a country mile and for both builds you're going to want to use Shattering Blow over Drifting Illusions because at 2 it does Groggy and at 3 it increases Crit Damage by 250%
DPS wise, in Total War or Tower if you pair her with Super Armor breakers like Electrocute Rin or White Feather Maika, she does more with the groggy build since that builds biggest problem is getting enemies stunned or groggy. If you go all out with her against a stunned enemy you can start hitting for 1 mill per melee hit. You'll need New Year's Inage, Vacation Lydia and Band Kurenai or whatever Debuff supporter you like.

Crit build is better if you're just throwing her into a pile with no planning, You'll need LAN to keep the cooldown of Shattering blow to a minimum, Suruga and Tekkain Kaworu for Crit Damage. You can drop Lan for Assault Rin/Lefia but you're mainly spamming Shattering blow so there's no reason to worry about criting outside of that.
I prefer her Groggy build because doing 1 mill on a normal attack is a lot better than 6-10 mill on a cool down skill but as always: Inage is expensive for no goddamn reason.