Peony is +Hp -Spd, Mirabilis +Def -Atk.
I already have both on neutral IV-s (Mira was 41st pull), and to be able to follow various guides neutral unmerged Peony is better. Mirabilis prolly won’t figure in the guides so often, but you never know.
Peony – Should I keep or fodder? (fodder what? and to which unit?)
Mira – Keep, fodder, or merge? I don’t have any other units with her A-B-C skills except Idunn (and she stays right where she is), but are those skills so good
For the favor of keeping them is also that sometimes in chain challenge my original team gets killed (for various reasons), and to be able to copy the first team saves lot of trial-and error.
I am not sure that 2 Peonys or Miras in the same team is very helpful, 2 x Eir at least heals 10 +10, and both can be danced.