That's a tough one. I don't really care much for either of them. To me, Hinata is too soft to be a Shinobi (let alone a Hyuuga Heiress) and her improvement isn't consistent with her character. She has always carried an air of meekness and incompetence with her; not a good look.
Karin...well....she's obsessed with Sasuke and aside from being a living healing circuit and a tracker, she doesn't really DO anything (at least from what I've seen). She is a ninja and I'd like to see some combat from her, maybe some jutsu. I haven't seen anything from her.