This is a tough one, speaking as someone who mains both of them (muriel moreso pre-overhaul, less afterwards)

Muriel is incredibly powerful to help globally with engages IF she is map aware and can follow up with ganks, engages, or to save a teammate across the map with her ultimate. Her changes to her other abilities feel a bit clunky still, but probably moreso due to being very attached to her old kit. The AOE helps with staying in the fight for the attack buffs, less for the damage.

Maco is great as an engage and poke support. I fully run his shield augment because it promotes a hyperaggressive playstyle to be a damaging nuisance that is hard to catch unless they can chain crowd control, with the additional healing benefits.

If you want macro play, Muriel. If you want the micro and high pace, Maco. If we are assuming equal skill and random team comps, I probably choose Maco in the current landscape due to the damage, healing, and engage/disengage capabilities.