Stages (even alternate stages) and actions (even optional actions, be they case wide or local) are two very different things.
Stages are a way to organize actions into a sequence. In the first stage, these actions execute; then, in the second stage, these actions execute, and so on. Stages are considered to be primary or alternate. Primary stages are used to organize actions that take place in the normal sequence of the case life cycle. Alternate stages are used to organize actions that take place outside the normal sequence of the case life cycle. Alternate stages are typically used to organize those actions that are considered exceptions to the normal flow of the case life cycle.
As to when to use an alternate stage and when to use a case-wide optional action.
If you place an action in an alternate stage, the flow of the case life cycle would have to move to that alternate stage for that action to execute. This requires you (the case designer) to account (design) for all possible entry points into - and out of - the alternate stage. The key take away is: the actions in the alternate stage cannot be executed until the case moves into that alternate stage.
A case-wide optional action is a design consideration that enables the participants in the case life cycle to decide when to execute that action - regardless of where in the case life cycle they are. The key take away is: you (the case designer) do not have to account for all possible entry points to the optional action; the action is available during the entire life cycle of the case - as an option.