Guardian

This rune is good for people who are just starting out with Milio. This ensures you're not too squishy, ​​making positioning mistakes more forgiving.

Guardian is also a good rune if you see that there are champions in the enemy team that will give you a hard time on lane. This rune is most useful against botlane combos like: ( and or and etc).

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🔥PROS & CONS 🔥
+ Forgives you positioning mistakes.
+ Stronger shields than Summon Aery
+ Makes more tanky.
+ Access to Revitalize rune.



- Your trades will deal less damage due to not having Summon Aery.
- Less ability haste than the Summon Aery.
Summon Aery is the alternative keystone and also my favorite rune for Milio you could take. The damage part of it is helpful in laning phase as Milio can poke the enemies with his Q extremly easily.

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🔥PROS & CONS 🔥
+ Summon Aery is good for early poke with Q and later on the shields.
+ It's good to take it on a lane where you can freely trade without dying.
+ Trades will be stronger.
+ Higher CDR's than Guardian because of Transcendence



- Squishier than Guardian because you get any defensive stats from your items/runes.
- Smaller shields than Guardian

Summary Take Guardian for more heals and shields. for more utility with Shurelya's Battlesong and more early damage with ur Q.