5* Zephiel or 5* L'Arachel?

+ATK L'Arachel is pretty fun. 59attack on full hp unit if you get Death blow. Res Tactic works well on mix team with DC units too.

I'd personally do Zephiel since he has a legendary weapon refinement and can be good unit to be used in, and even outside, of Armor Emblem

L'Arachel is a good choice both with her base tome and a Bladetome, especially with her being +Atk.

That’s so weird, I just decided to promote both L’Arachel and Zephiel today. Both are solid choices but I think you’ll have more fun with L’Arachel built up. Good luck

Depends heavily on available fodder. If you have Distant Counter and some divine dew go with Zephiel, have Swift Sparrow then you can run L'Arachel (I just wouldn't run her with Blarblade, she prefers +spd for that).
If you have all that then it depends on what the respective emblem team needs; are you missing a capable Red Armor or a Blue Horse Nuker?