First instinct is teeth for hair, because hair for teeth means constant upkeep to keep from choking, and can't chew. But then I remembered laser hair removal exists, and so do dentures.
Now my main wonder is if body hair and pubic hair is also replaced with teeth... I think I'm leaning more towards hair for teeth. Being covered in patchy, sparse (potentially curly?) teeth under my arms and all over my body sounds... Worse. Much worse.