1. Key Difference 1: Purpose and Focus: Google Cloud Container Builder is primarily focused on providing a cloud-based solution for building and testing container images, along with the integration of other Google Cloud services. It offers managed build infrastructure and seamless integration with various container registries. On the other hand, Harbor is an open-source container registry that focuses on providing a secure and scalable environment to store, sign, and distribute container images. It offers features like vulnerability scanning, image replication, and role-based access control (RBAC), making it ideal for organizations requiring advanced security and compliance capabilities.

2. Key Difference 2: Deployment Flexibility: Google Cloud Container Builder is tightly integrated with Google Cloud Platform (GCP) and is primarily meant for building and deploying containerized applications on GCP. It seamlessly integrates with other GCP services like Google Kubernetes Engine (GKE) for easy deployment. In contrast, Harbor is a platform-agnostic container registry that can be deployed on-premises or in a cloud environment, providing flexibility to organizations to choose their underlying infrastructure and deployment approach.

3. Key Difference 3: Management and Maintenance: Google Cloud Container Builder is a fully managed service provided by Google, which means all the underlying infrastructure and maintenance tasks are handled by Google. Users can focus on their application development and let Google take care of the build environment. On the other hand, Harbor being an open-source project requires manual setup, configuration, and maintenance. Organizations need to allocate resources and manage the infrastructure to run Harbor, including updates, backups, and security patches.

4. Key Difference 4: Extensibility and Customization: Google Cloud Container Builder provides a set of built-in builders and build steps that cover most common use cases. However, it also allows users to create custom builders and add custom build steps using custom build templates, providing flexibility and extensibility. In contrast, Harbor offers a pluggable architecture that allows users to extend its functionality by integrating with third-party services. Users can add custom authentication methods, implement custom replication policies, and integrate with external vulnerability scanners, making it highly customizable.

5. Key Difference 5: User Interface and User Experience: Google Cloud Container Builder has a user-friendly web interface along with a Command Line Interface (CLI) that allows users to easily configure and manage builds. It provides real-time logs and notifications, making it convenient to track the build progress. On the other hand, Harbor also provides a web-based user interface for managing container images, projects, and users. It offers role-based access control, search functionality, and a comprehensive dashboard to monitor the overall health and status of the registry.

6. Key Difference 6: Community and Support: Google Cloud Container Builder benefits from the extensive Google Cloud community and support, which ensures regular updates, bug fixes, and continuous improvement of the platform. Users can rely on Google's technical support for any issues or queries they face. On the other hand, Harbor being an open-source project, has a vibrant and growing community that actively contributes to its development and maintenance. Users can leverage community resources like forums, documentation, and community-contributed plugins for support and additional functionality.

In Summary, Google Cloud Container Builder is a cloud-native build service tightly integrated with Google Cloud Platform (GCP), while Harbor is an open-source container registry that focuses on security and scalability. Container Builder provides managed build infrastructure and easy integration with GCP services, while Harbor offers advanced security features and deployment flexibility.