Lorde's kinda creepy looks like she'd summon Satan to burn Ariana with hellfire, but Ariana's eyes be serving "Orphan" soulless teas sometimes so she might go beserk and beat Lorde down.
Lorde would go bat **** crazy on Ariana. She would have the strength of 20 demons on her side. However, Ariana does strike me as the type to have some fight in her. If this was like Ariana and Demi, for example, it would be harder to decide. But Lorde just seems to crazy for her.