1. Accessibility: Postman is a comprehensive API testing tool that allows users to test APIs directly within the app, whereas Shields.io is a service for generating badges for various purposes, primarily to display information in a visual form on websites and GitHub README files.

2. Customization Options: Postman offers advanced customization options for tests, environments, and workflows with features like scripts, variables, and test assertions, while Shields.io focuses on generating badges with limited customizability in terms of design and information displayed.

3. Real-time Collaboration: Postman allows team members to collaborate in real-time on API development and testing, with features like shared workspaces and version control, unlike Shields.io, which is primarily a self-serve badge generation service without collaboration features.

4. Automation Capabilities: Postman provides robust automation capabilities through its collection runner, Newman CLI, and monitoring features, enabling users to automate API testing and monitoring workflows, whereas Shields.io does not offer automation features as its main function is badge generation.

5. Offline Work: Postman offers a desktop application for users to work offline and sync their work when internet connectivity is available, ensuring flexibility in use, whereas Shields.io operates solely as an online service, requiring internet access to generate and display badges.

6. Extensibility: Postman supports integrations with third-party tools and services through its extensive library of integrations and APIs, enabling users to extend the functionality of the tool, which is not a feature provided by Shields.io, as it focuses on badge generation exclusively.

In Summary, Postman excels in API testing and collaboration with advanced customization and automation features, while Shields.io specializes in badge generation for visual representation on websites and README files.