1. User Interface (UI): Insomnia REST Client, Paw, and Postman have different user interfaces. Insomnia REST Client has a clean and minimalistic interface, with a sidebar for organizing requests and a code editor-like request builder. Paw, on the other hand, has a more visually appealing interface with a sidebar for organizing requests and a request builder that resembles a flowchart. Postman has a similar interface to Paw, but with additional features like collections and workspaces.

2. Authentication: Insomnia REST Client, Paw, and Postman support different types of authentication methods. Insomnia REST Client supports basic authentication, OAuth 2.0, and API key authentication. Paw supports basic authentication, HMAC authentication, OAuth 1.0, OAuth 2.0, and API key authentication. Postman supports basic authentication, OAuth 1.0, OAuth 2.0, and API key authentication, but also provides additional authentication methods like bearer token and digest authentication.

3. Extensions/Plugins: Insomnia REST Client, Paw, and Postman have different levels of extensibility. Insomnia REST Client provides a plugin system that allows developers to add custom functionality and integrations with other tools. Paw supports plugins and extensions that can be used to enhance its capabilities. Postman also supports plugins and has a wide range of available plugins that offer additional functionality like data generators and testers.

4. Collaboration and Syncing: Insomnia REST Client, Paw, and Postman have different collaboration and syncing capabilities. Insomnia REST Client and Paw provide collaboration features like sharing of request collections and environments with team members. Postman goes a step further and offers collaboration features like team workspaces and role-based access control. Additionally, Postman also provides syncing capabilities that allow users to sync their API collections and environments across different devices.

5. Automation and Testing: Insomnia REST Client, Paw, and Postman all support automation and testing of APIs. Insomnia REST Client has a built-in support for writing tests using JavaScript, which can be helpful for automating API testing. Paw offers a code generation feature that allows users to generate code snippets in different programming languages for making API requests. Postman also supports tests using JavaScript, and provides a visual interface for creating tests and assertions.

6. Price: Insomnia REST Client, Paw, and Postman have different pricing models. Insomnia REST Client is open-source and completely free to use. Paw has a free trial version, but it requires a license for full access to all features. Postman offers a free version with limited features, but also has paid versions with additional features and capabilities.

In summary, the key differences between Insomnia REST Client, Paw, and Postman lie in their user interfaces, authentication capabilities, extensions/plugins support, collaboration/syncing features, automation/testing capabilities, and pricing models.