One of the differences is that the Task/Template can be used to create many tasks via the Fulfillment Item Package and Task Catalog objects. Take a look at the workflow on the Fulfillment Item BO. It uses the Task/Template Instantiation block. Basically you can create a fulfillment item package with a set of tasks as a template, then create a fulfillment item (under a service request) that uses a particular fulfillment item package. It will (via the workflow) create all of the tasks that are associated with the package on the fulfillment item.