Which hero should I keep?

Pure power with death blow and att wave type skills=+att
Doubling with desperation=+spd

If you care about AR, you should keep and use all 3 Eirs in your offensive team. Otherwise, +res is better.

I would pick +atk Elincia and focus on raw damage instead of doubles. The +spd could be useful with firesweep sword in a flying team, but L!Ryoma does that better.

+Res eir because hp is not too important and is fine to lower
Elincia I would go with attack because if she is strong enough she won't need the extra speed.

+res is really good on Eir because she needs to keep her HP high to get the bonus stats from her weapon.

On Elincia I would recommend +atk unless you're concerned about missing galeforce due to OHKO. Anything that's super-fast probably falls in two hits; I wouldn't expect to need the extra spd except against slow tanks. Even if you run into some high spd/def unit (they do exist - F.Corrin, Selena, Subaki, Nephenee for instance) I don't think +spd -atk does better than +atk -spd.