Several declarative features can look similar while solving different requirements. A formula calculates a derived value, a roll-up summarizes related details, a validation rule rejects an invalid save, and a flow performs a process or record action. Choose from the required behavior and its lifecycle.
Ask when the value should change, whether someone should edit it, which records it depends on, and what should happen when inputs are missing. A stored copy can go stale; a live calculation can have reporting and permission consequences. Use the least complex supported design that you can explain and test. These scenarios ask you to defend the choice, including the relationship and access assumptions, rather than simply naming a favorite feature.