In Hell's Core fight, Mirajane could have blasted Sayla with her demon blast or other blast demon magic like she did in her vs Freed but she didn't do that because she was using her ethernano to activate her demon soul and to take over Hell's Core demonic tentacles at the same time. If she just had focused on herself to activate her demon soul, she anytime must have overwhelmed Sayla with her all blast spells.
Even if Elfman was not around them, Mirajane was still capable of activating her demon soul and transformed herself again to fight against Sayla, but she cleverly took over Sayla's Macro quickly and used it to control Elfman who was around instead of transforming herself. Maybe taking Macro over requires less MP than using her Sitri mode.
In Face Control Room, Mirajane once again transformed herself to smack Sayla to death. This can be considered a proof if Mira has more energy than Sayla.
Anyway, I still choose the boys as winner though