Key Differences
Architecture:

bull: Uses Redis as a backend, which makes it simpler to set up and use for job queuing and background processing.
Kafka: Requires a more complex setup with Kafka brokers and Zookeeper, designed for distributed, high-throughput messaging.
Use Cases:

bull: Ideal for background job processing, task scheduling, and handling asynchronous tasks in Node.js applications.
Kafka: Ideal for real-time data streaming, event sourcing, and building scalable, distributed systems.
Scalability:

bull: Suitable for small to medium-scale applications. Limited by the scalability of Redis.
Kafka: Designed for large-scale, high-availability applications. Can handle millions of messages per second.
Complexity:

bull: Easier to set up and use, especially for Node.js developers.
Kafka: More complex to set up and manage, but offers more powerful features for distributed messaging.
Example Use Cases
bull:

Background job processing (e.g., sending emails, processing images).
Task scheduling (e.g., cron jobs).
Rate limiting and job prioritization.
Kafka:

Real-time data pipelines (e.g., log aggregation, metrics collection).
Event sourcing and CQRS.
Stream processing (e.g., real-time analytics, monitoring).
Conclusion
Use bull if you need a simple, easy-to-use job queue for background processing in a Node.js application.
Use Kafka if you need a robust, distributed streaming platform for handling large volumes of data and building real-time data pipelines.