How would you decide between an automation rule, a completion action, and an Engagement Studio action to apply the same change to a prospect?

They differ in what triggers them and how often they can act, so I pick based on the trigger, not the outcome.

A completion action is the right choice when the change should happen the instant a prospect interacts with a specific asset — submits this form, clicks this redirect, downloads this file. It is scoped to the asset and needs no ongoing evaluation, which makes it cheap and easy to reason about.

An automation rule is right when the condition is about the prospect's overall state rather than a single interaction — grade and score crossing a threshold, a combination of field values, membership of a list. It evaluates the whole database continuously, which is powerful but consumes one of a limited number of active rules, so I do not spend one where a completion action would do.

An Engagement Studio action is right when the change belongs to a sequence — the prospect should be scored up only because they reached step seven of this specific nurture, and the context of the programme matters.

The practical failure I look out for is the same change implemented in two places, which makes scores drift and is painful to debug months later. One trigger, one owner.