Originally Posted by
Jeeqbit
Shields can absorb DoT ticks and mitigate percentile attacks (like most of Eden's raid-wides). A powerful enough shield can prevent some effects such as knockbacks or Doom (depends on boss). Crucially, they can be used after the cast is completed for things like DoTs.
Invulns work if used before the cast completes but work after the cast for things like DoT ticks. Invulns are hallowed ground, holm gang, living dead and superbolide.
Raw % mitigation is only useful before the cast completes in most cases, as it snapshots at the end of the cast. Raw mitigation can reduce the potency of DoT ticks as well if used before cast completes, which was super important in the Abyssos raids. This includes Reprisal even though it's more of a damage down on the enemies.
Healing instead of mitigating is valid (ie. Warriors do it with Thrill+Equibilibrium), but only if the lack of mitigation won't cause them to die or nearly die (usually they won't actually die to lack of mit if healed, but there are some exceptions, especially if gear isn't good or not max HP).
In summary, most of the time it doesn't matter which, but understanding these nuances will help a bit.