Thanks for using my guide! I would say that it is preference since there are a few challenger players that run Sunfire. I personally like Frostfire more because of the mythic passive which grants all your other legendary items extra health. I also have some notes under how I feel about which mythic item to take under the item description. That's my take! Happy your playing Zac, he's a great champ in all elo's. AP Zac mid can be a monster late game and very fun to mess around with!