This was really tough to decide, but I vote Sakura. Honestly the only reason she's losing in the poll is because it's sakura, people will hate on her no matter what.
In my opinion, realistically Sakura is who you would want on the battlefield next to you. The main fact being that she can actually fight and has incredible power, compared to Karin, whose chain ability is quite unpredictable and untrained.
Karins healing ability is superb, but Sakura also has Katsuya, so she can provide her team with a small slug/Katsuya and heal remotely, while her teammate can still fight without worrying about chakra or injuries. That's very useful, compared to having to stop everything and bite karin, that makes both Karin and her teammate vulnerable. Katsuya is also very useful in battle, she can spit acid and all sorts, so that provides for better support and offensive use.
Also with Sakuras Byakugo, she's incredibly difficult to kill, and can heal and recover from most wounds and the most fatal of injuries. Karin is quite an easy target for the enemy. At least Sakura is strong enough to hold her own.
Karin's sensory ability is the only thing that would be a big miss as it is very useful for detecting all sorts. However it can in a way be compensated, Katsuya can split up into thousands and spread out across a wide scope of the battlefield, in order to keep an eye out.
At the end of the day, you'd fear for Karins safety more than I would Sakura. Plus you don't need to be anywhere near Sakura to receive healing (Katsuya in your pocket ftw), meanwhile having to bite Karin leaves me vulnerable.Theres hardly need to babysit Sakura compared to Karin, even when Sasuke stabbed her, she was too weak to hear herself, Sakura won't have that problem, she heals pretty much automatically.