Event Receivers vs Workflows

Event Receivers	Workflows
Can execute either before or after the operation	Always executes after the operation
Can cancel the operation	Cannot cancel the operation as the item will have already been created
Cannot be launched manually by the user	Can be started manually by the user
Not designed for user interaction	Specifically designed for user interaction
Can execute in response to many different actions, including deletes	Can only execute in response to one of four events – Manual, an item being created, an item being edited, and an item being submitted for approval
Must be created in Visual Studio	Can be created with Visual Studio, Visio or SharePoint Designer

The main key when choosing between a workflow and an event receiver is the need for a UI. If user interaction is required to process an item a workflow is almost always going to be your best choice. If you need to respond to an event other than the four allowed for by a workflow, or the operation is completely automated, then an event receiver is the best option.

Credits: http://blog.geektrainer.com/2011/03/event-receivers-vs-workflows.html