I vote guild coins. Changing the guild quest rewards to event resources during events is a bad suggestion and it would unbalance the game, making it unfair to players not in a guild or active players in a not very active guild that can't complete their guild quests.
It should be said that being a member of a smaller but fairly active guild that would probably benefit from the proposed change, I still vote against it because I am more concerned with the overall balance and growth of the game.