Probably best to think about what you like about a hero and how you use your heroes.
Yes, having another 5* Ninja does make a difference. Do you like spirit link? Mica does no damage and puts no pressure on the enemy but has the potential to give a huge mana boost and defense up. Would your playstyle allow you to wait until the third charge often?
Sif does not do any damage but closes off part of the board to the enemy and her damage reduction is immensely helpful. Can you work her into your team? She really makes slow heroes shine (no mana troop they go from 12 to 10 tiles, slow to average mana, with mana troops and emblems you need +10% more mana to go to 9 tiles on a slow hero, that is functionally fast), do you have any slow heroes that you use when raiding? If so, she will protect them and boost them.
Uraeus is good while minions are everywhere. Average mana with a decent hit. I haven’t used him nor faced him (I don’t expect to face him in defenses much either).
I love Sif and she allows me to use MN. I would like Mica but don’t know how I would build around him. Uraeus is too soon to say beyond reading the card.
This is still beta that we are all using now, DON’T choose a hero based on this beta version, wait until the final version of formations is made public. If they change the impact of nearby on the middle position if radically alters the relative strength of many heroes (looking at you Sif) and a decision based on beta could be painful.