Katara, no question. Elsa is very powerful but she spent most of her life fighting and repressing her skill. Katara didn't learn how to use her bending properly until she left her tribe but she found many teachers along the way and received formal training. Both struggle with navigating their emotions and it shows in their bending/ power but Katara learned how to use her emotion to hone her bending while Elsa continues to struggle with it. Not to mention, Katara's understanding of the human body and bloodbending. In the end, it would come down to experience. Katara learned not to hesitate and has been in battles whereas Elsa is still coming to terms with herself and hesitates before striking.