Which Eir?

That depents on how much you use the F2P guides.
Those depent on the neutral IV
But +Res is a better IV in general.

+res is really nice. It helps her maintain her weapon's effect.

My Eir has the same IVs and Res+ is a nice boon. You can give her flying bonuses to increase her res and other stats too.

How about keeping and using both in AR?

You can get 120 lifts per battle with 2 Eirs and 2 Light blessed units, or 140 lifts if one of them is a bonus unit.

Use the +RES as healer bait.