For everyone who thinks Fel is stronger consider this. As a Titan closely aligned with Arcane Sargeras made short work of every Demon infested Fel touched world.Sure he didn't know how to kill him but that didn't stop him from jailing them in Mardum, a Dimensional Prison. The only reason the fel magic of the Legion is so strong now is because Madrum became overcrowed and boiled over when when Sargeras unleashed them to go Baby Titan hunting.
I think this explains quite clearly that Arcane and Fel are evenly matched however Sargeras tipped the balance when he created Mardum.