>>20032377E1 is basically a permanent 12% SPD boost which is really comfy. E2 is just a revive I would say only E6 a character if you really like them.
If you want a general idea on how good a character's Eidolons are, you can go to this site, pick your character, and scroll to their Eidlon section to get a grasp.
https://game8.co/games/Honkai-Star-Rail/archives/427878