I'll match you raise you a grand!!

But, in battle, it depends on my situation. If it was one where it'd simply be dificult to survive, I'd pick the better healer (Eiko), in a fight when i couldn't be arsed to take them properly, I'd summon something tough with Garnet.

My main reason for preferring Garnet, is because she can summon leviathan, the water demon. I prefer water spells to any other. (Gizmaluke is my fave boss because of this)

I like them both. Garnet is better developed, but a lot of people don't realise that Eiko isn't just a little brat. She's been through a lot too, and she's hella mature for her age. (I really wish she was about 10 or 11, that would be more believable.)

Garnet: The majority of her summons are obsolete by disc for. The only good ones are Ark, Bahamut and Odin (even better when equipped with Odin's Sword). My Odin is actually more powerful than Bahamut, but I suppose that's because I have more Ore (is it Lapis Lazuli that powers Odin?) than the number of Garnets. However, Eiko's Madeen and Holy are effing powerful. Many monsters are weak against Holy damage by disc four, so Eiko excels. Against a Hecteyes, Holy does 9999 damage, while any of Garnet's Eidolons do 4000 damage, max.

Sure, Garnet's Odin is really useful when it kills all the enemies, but that hardly ever happens. Garnet's Eidolons are about power and not strategy. Eiko's are really useful when against bosses. She can use Carbunkle to reflect everyone. Then, you have Vivi with ReflectX2 to use Water on the party, and you have an instant 9999 damage on the enemy.

Even a reflect on a party member and then casting Holy on that party member does a lot of damage. If a party member is knocked out, Eiko can use Full-Life. Garnet only has Life. If multiple members are KO'd, Eiko can use Phoenix. If everyone is knocked out, but more specifically if she's the last one to get knocked out, there's a chance she'll auto-summon Phoenix to resurrect everyone. In Trance, she could Double-Holy. Lots of damage

Eiko also gets Curaga waaaaay before Garnet does. Garnet's Eidolons are powerful though, when she's levelled up enough, and when you have the number of stones to make her more powerful. In Trance mode, using Garnet's "Eidolon" move to summon an Eidolon will do damage to an enemy. Then, on every one of Garnet's turns, it will auto-summon, doing damage. Garnet gets another turn because the auto-summon doesn't cancel out her turn.

It's a tough one, but I like them roughly the same.