Do you want a sexy fire dragon lady that makes people stronger(attack and defense buff) or a giant wind loli that cures some status effects.. both in addition to their relatively heavy attack. Both passively boost elemental affinities if they're your primary/support summon - Behemoth boosts wind, light and water; Fafnir boosts fire and water.
Keep in mind that it's fairly easy to get SR Eidolons of all elements.