1. API Documentation Support: ReadMe.io is specifically designed for API documentation and provides advanced features such as interactive API consoles, SDK generation, and API mocking capabilities. On the other hand, Read the Docs is more focused on general-purpose documentation for software projects and lacks the specialized API documentation features of ReadMe.io.

2. Built-in Version Control Integration: Read the Docs seamlessly integrates with version control systems like Git and Mercurial, allowing automatic documentation updates based on code changes. ReadMe.io, on the other hand, provides a more manual approach to updating documentation, requiring users to manually upload and update documentation files.

3. Content Management Capabilities: ReadMe.io offers advanced content management features such as rich text editing, images, and videos embedding, and customizable themes to create visually appealing and engaging documentation. Read the Docs, while functional, has limited content management capabilities, primarily focusing on straightforward text-based documentation.

4. User and Developer Support: ReadMe.io provides robust user feedback and engagement tools such as comments, ratings, and analytics to gather insights and improve documentation based on user interactions. Read the Docs, while offering basic user feedback options, lacks the comprehensive user engagement features of ReadMe.io.

5. Collaboration Features: ReadMe.io enables collaborative authoring by allowing multiple team members to work on documentation simultaneously, track changes, and manage permissions. In contrast, Read the Docs is more suited for individual contributors or small teams, lacking the advanced collaboration features offered by ReadMe.io.

6. Customization and Branding: ReadMe.io allows extensive customization and branding options, including custom domains, logo integration, and CSS customization to align documentation with a brand's style and identity. Read the Docs, while customizable to a certain extent, does not offer the same level of branding and customization options as ReadMe.io.

In Summary, the key differences between Read the Docs and ReadMe.io lie in their focus on API documentation support, version control integration, content management capabilities, user and developer support, collaboration features, and customization and branding options for documentation.