meh. X factor for me is a variable which may have the biggest impact in the game. Nate's offense is definitely a variable which gives the Bulls a huge advantage when it's on video game mode wherein he makes absurd shots.
but yeah, sure, Butler is a also a significant factor in this series. His defense on James in the first half was impressive plus his 48minutes on the court was definitely a key factor.
I still give it to Nate but both players are definitely key players in this series.