When to use an Event Handler rather than a Workflow

In general, you will want to create an event handler rather than a workflow when you need to attach small bits of code that will run immediately before or after a list event has occured.
 As a rule of thumb, if your event handler must perform more than one or two operations (for example, updating some fields, validation data, or sending an email), you should consider using a workflow rather than an event handler.

Quoted from nice and easy to read book
SharePoint 2007 Development Recipes: A Problem-Solution Approach (Expert's Voice in Sharepoint)