Garnet for a ton of reasons. Not only was Eiko just flat out annoying but Garnet is one of the best developed characters in the entire game. I won't ruin the game for anyone, but she definitely was one of the best thought out characters of any game in the series. Not only that, she wasn't bad on the battlefield. Granted I'm not much a fan of healers but she was pretty useful.
I found Eiko to be an annoying brat 
At least Eiko gets to use double white magic

