Whats better for head lice and nits: apple cider vinegar vs. tea tree oil?

Neither: Nits are the eggs of head lice, so you are right to seek a treatment that kills both. Vinegar and oil tend to fail. Pesticides can be toxic. We recommend harmless interventions, like forming a silicon shell on the head with applications such as licemd.

Not recommended: Tea tree oil (melaleuca) is purported to be useful against fungi and bacteria. But there is no scientific evidence it is helpful for those or head lice. And if oxidized and applied on skin, or ingested, it can be quite dangerous to children. Vinegar has no effect on nits or lice. Better to consult your pediatrician for a safe an effective treatment.