Normally if you were asking me to pick between those two characters, I'd say Quina as I never really found a use for Amarant. However with that particular squad, I'd go with Amarant. You should have your magic attacks covered between Garnet and Eiko, so you'll want to more offensive attacks from Amarant.

Though I agree with Quin that if you're playing without Steiner, you might as not bother playing at all!