Your Fire team will likely be lacking ATK Down, and you'll be using Mordred with Ambush most of the time. It'll have very high damage potential.
Your Water team will be better balanced. You'll have stronger heals, a cleanse, combo attack down, and you'll likely be using Mordred with Sniper Shot most of the time. You'll have access to full debuffs and high base damage due to capped DEF down. Access to Jorm as Friend eidolon means you'll have nearly capped ATK down too.
Water seems like the stronger option to me.