Deciding when to use a workflow vs. a plug-in is usually a straight forward process. In general, I always recommend using workflow to automate business processes especially if the business process will be later modified/maintained by “business” people (non IT).
However, there are some characteristics that will guide you to use one versus the other that do require you to know a bit more details of the task that you are trying to achieve. For example, if you require performing an action right away, let’s say when a record is updated (synchronous), then you have to use plug-ins because workflows are always asynchronous.
The following table gives a quick comparison between workflows and plug-ins and when to use each one, hope you find it useful.