You generally don't want to use those 30% Atk / Ability DMG↑ event eidolons as mains as that passive effect is awful. The weapon is better in the long run and if you can break limit it 3 times, then you'll be able to max it since you only need to do so 3 times for max break while eidolons need one more.

In a month or so, the next event water eidolon Tiamat will be available and she has very good stats (highest atk of all event eidolons), a useful skill and a good enough passive to warrant using as a main eidolon for a free player.