On my free summon I got a +atk,-res, but I already have a +atk,-def. Which elincia is better?
On the rare occasion she gets put into a fight where she can't immediately pop the enemy, it'd be best to keep her Defense normal to save her some damage, since they'll most likely hit back with physical damage unless you put her up against W!Tharja or another Close Counter mage.
I would suggest keeping her defense so she doesn't take too much damage from units like Hector (both of them). I have a +ATK, -RES Elincia myself and she tears through green units like tissue paper. I don't run ploys on mine and I want to keep her physical bulk. Personally, +ATK, -RES is my preferred choice.