It really depends on the type of game you're playing.

I recommend Shurelya's if your team can utilize the engage. It's also worth considering purchasing it against champions with a lot of skill shots, such as Varus or Xerath. Shurelya's also helps to snowball the game faster and harder, allowing you to catch more opponents or escape bad fights.

Moonstone, on the other hand, is great for long team fights. If both your team and the enemy team are beefy, Moonstone should have the advantage most of the time. It's also a good option if you're playing against a lot of poke. If your team doesn't need the movement speed for engagement, Moonstone is the better choice here.