Mythic key = can open event chest one time; BUT you might as well spend 2000 coins to open event chest.
AND it seems you cannot even save it for next girl event. 2000 coins usage is much more flexible - can be used for x10 pull (18,000 coins) BUT not this key. Unless this Mythic Gold KEY is intended to be a guaranteed Mythic draw, I do not consider this item as a better reward.
Pirate coin may as well help you in event goal too. So, for example i will definitely feel penalized in case if I get to daily rank 10 instead of 11.