I prefar Katara because she is more mature than Korra although she is younger: she was in charge of an entire Tribe with his 16-year-old brother and she never complained. Also, she is a great warrior that started bending without a teacher and finally, without her, Aang wouldn't have done the things he has done.
Katara. I hate Korra; she's your typical annoying teenage female anime character - and yes, I KNOW Avatar isn't an anime, but she still fits that mold. She thinks the world of herself. She's cocky, selfish and refuses to listen to reason. I assumed she'd improve, but no... she has shown barely any signs of character development whatsoever. She's still the same old hot-headed brat. I guess she could be worse... but that doesn't make her better.
Oh, but Katara is lovely. :) She's never done anything to annoy me, and she's pretty darn cool! Not to mention kind.