Kafka Streams is a Java library for building complex streaming applications with flexible logic and integration with external services. ksqlDB provides a declarative SQL approach for stream processing and is better suited for typical ETL tasks and rapid development. If you need complex business logic, API integration, or custom transformations, Kafka Streams is preferable. For simple aggregations, filtering, and join operations, ksqlDB is more convenient. The choice depends on the complexity of processing required and the need for integration with external systems.