Since the asker never specified a format, I guess I'll assume he was statistically likely to be asking about OU or VGC18. In OU, the most common Pokemon include Magearna, Kartana, and Ferrothorn. In VGC, the most common Pokemon include Metagross, Kartana, and Aegislash. No fighting type was as common as any of those. Therefore, in both OU and VGC18, fire is the better coverage type.