1. Key Difference 1: Integration with Other Tools Gitbook offers seamless integration with other popular tools such as GitHub and Slack. Users can directly link their Gitbook repositories to their GitHub accounts, allowing them to easily manage and sync their documentation. On the other hand, ReadMe.io also provides integration options, but it primarily focuses on the API documentation and developer experience. It offers features like auto-generating API documentation from code and API explorer, which enhance the overall API documentation experience.

2. Key Difference 2: Collaboration and Version Control Gitbook provides powerful collaboration features that allow multiple users to work on the same documentation project simultaneously. It offers features like real-time editing, commenting, and change tracking, enabling teams to collaborate effectively. In contrast, ReadMe.io offers version control functionalities specific to API documentation. It allows developers to document different versions of their APIs, collaborate with team members, and manage API changes effectively.

3. Key Difference 3: Customization and Theming Gitbook provides extensive customization options, allowing users to personalize the appearance and layout of their documentation. Users can choose from a wide range of themes, customize the CSS, and add their branding elements. ReadMe.io, on the other hand, focuses more on simplicity and ease of use. It provides a clean and user-friendly interface with limited customization options. However, it ensures the documentation looks consistent across different devices and screen sizes.

4. Key Difference 4: Pricing and Availability Gitbook offers both free and paid plans, making it accessible to individuals and organizations of all sizes. The free plan provides essential features, while the paid plans offer additional functionalities and advanced collaboration options. ReadMe.io follows a subscription-based model, where users need to pay a monthly or annual fee to access its features. It offers multiple pricing tiers based on the size and needs of the organization.

5. Key Difference 5: Extensibility and Integrations Gitbook provides a wide range of integrations and plugins that enhance the capabilities of the documentation platform. Users can integrate with tools like Google Analytics, Disqus for comments, and add custom plugins for specific functionalities. ReadMe.io, on the other hand, offers a limited number of integrations and extensions. It focuses more on providing a streamlined documentation experience without overwhelming users with too many options.

6. Key Difference 6: Target Audience and Use Cases Gitbook is suitable for a wide range of use cases, including software documentation, project documentation, and knowledge bases. It caters to both technical and non-technical users, making it versatile and flexible for various industries. ReadMe.io primarily targets developers and organizations building APIs. It focuses on providing an intuitive and comprehensive experience for documenting APIs, SDKs, and developer guides.

In Summary, Gitbook and ReadMe.io differ in integration capabilities, collaboration features, customization options, pricing models, extensibility, and target audience.