Mercedes: Critical % vs. Damage/ATK %?

well I'm trying to figure out what's better for 250k max range. still cubing.

and %dmg is applied after everything else, so I think of it more as +x% to all skills

just that i have heard that %damage stacks additively with mercedes skills, but i don't have one of those 89% bossing arrows to go and check math, so it's just a rumor for me.