Both are S-tier. Trust me. Rainbow Rose's Damage is ridiculous.
Kerria has better Survivability. Rainbow Rose deals way more Damage in most Fights (she can deal 800k if she attacks twice and is full ampuled, Super Ampules not included.)
If you build your team around Boss Killer Girls i am sure the team can obliterate every boss in one turn if the girls are full maxed (Equipment Slots and Skill Levels included)
If the enemy attacks first and is able to oneshot your team obviously Kerria is better cause she has dodge.