None, they're both trout. I hate both of these characters.

Seriously, though, Amarant. Garnet has her summons, but isn't a huge damage dealer. Eiko is a healer, and except for Holy and Madeen, doesn't really do great damage. Really, she should be utilized for White Magic only, basically. I'm assuming Zidane is your third member. He does some great damage (although Steiner does better.)

To keep your party balanced, I'd say Amarant, because he can take a lot of damage, and he can DEAL a lot of damage. His throw ability with weapons (and at this point you should have tons and tons of gil, so you can go and buy a ton of high-power weapons) is devastating.

Quina is a good healer, and CAN Do good damage. But you have to spend hours getting his Blue Magic, or catching frogs. Quina is extremely weak, even with good weapons. He is even more susceptible to being hit, no matter how high of armor you have on. Because you're going with two casters who have low defenses, I'd advises against choosing Quina, as your party could go down quickly.

Hope this helps!