In the business requirements analyst’s world, process models and use cases both describe the ways in which people achieve goals or respond to events.
One way to figure out which technique to use is to apply the “coffee break” test.
Process models show how a goal is accomplished end-to-end across an organization, and shows how all of the groups in that organization interact. The timelines associated with process models are often measured in days, but may be much longer. If the work you are describing has many groups working on many items for many days you should use a process model.
Use cases show how a individual (actor) accomplishes a goal and is written from the perspective of that individual. Use cases correspond to activities within a process model. If you can complete the activity before you get up and have a coffee break then you should use a use case.
Process models and use cases are both part of documenting business requirements.