Better Weapon Upgrade Candidate: Ephraim or Eldigan?

I finally farmed enough Divine Dew to either upgrade Eldigan (+Atk/-Spd)'s Mystletainn (+Eff) or Ephraim (+Def/-HP)'s Siegmund (+Eff). My Eldigan is +2 and could reach some crazy BST and I already have B!Lucina, but which would be more worthwhile to upgrade?