Should I upgrade kuhuna to spirit walker or witch doctor since I have 0 5 star-able healer as of now....
Personal Opinion
Both are kind of bad TBH.
Active Skill
Spirit Walker has 5T Single HoT + reflect damage - Not so bad
Witch Doctor has 6T Party HoT + Reduce healing of enemy (PvP skill) - Very bad in PvE content
Aura
Sage Path has burden Aura path, Because their Aura is trigger type and it doesn't works with alone.
Spirit Walker - When others in Aura gets healed there's a 30% chance for them to attack with 80% of their attack - this unit have single healing, when you have no other persist healing units this Aura is basically a useless Aura with only 30% trigger rate. Yes you can activate his active every 5T but remember this, you can only heal 1 unit and there's only 30% chance to active.
Witch Doctor - When Hero gets healed there's a 40% chance for others in aura to attack with 80% of their attack - this unit needs healing to trigger like any other sage units, if you were to use him alone. He will have a 40% chance to trigger every 7T, so basically you need another persist healing unit to utilize his aura.
So basically in my opinion, all sage class is a burden to any team. unless you have another healer in the team, if not they can't fully utilize their aura.