1. Flexibility and Customization: Webiny Headless CMS offers a highly flexible and customizable architecture, allowing developers to build and customize their own content model and APIs. On the other hand, concrete5 provides a more structured approach with pre-defined content types and templates, limiting the level of customization.

2. Headless Approach: Webiny Headless CMS follows a headless approach, separating the presentation layer from the backend logic. This allows for greater flexibility in choosing the frontend technology stack and enables developers to create highly performant and scalable websites. In contrast, concrete5 is a traditional CMS that tightly couples the frontend and backend, limiting the flexibility in using different frontend frameworks or libraries.

3. Scaling and Performance: Webiny Headless CMS is designed for high scalability and performance, making it suitable for projects of any size. It utilizes serverless architecture and provides built-in support for caching and CDN integration. On the other hand, concrete5 may face challenges when it comes to scaling and handling high traffic, especially in larger projects.

4. Out-of-the-box Features: concrete5 comes with a range of built-in features, including a user-friendly interface, drag-and-drop functionality, and a marketplace for add-ons and themes. Webiny Headless CMS, on the other hand, takes a more minimalistic approach and does not provide as many out-of-the-box features. However, it offers a wide range of integrations and APIs, allowing developers to extend the functionality as needed.

5. Learning Curve: concrete5 is known for its easy-to-use interface and intuitive editing experience, making it a popular choice for non-technical users. In contrast, Webiny Headless CMS has a steeper learning curve, targeting developers and technical users. It requires a solid understanding of web technologies and GraphQL, which may be less accessible for non-technical users.

6. Community and Support: concrete5 has a large and active community, with a wealth of resources, tutorials, and forums available for support. Webiny Headless CMS, being a relatively newer platform, has a smaller community and may have fewer resources available for troubleshooting or getting help.

In Summary, Webiny Headless CMS offers greater flexibility and customization, follows a headless approach, provides high scalability and performance, has a minimalistic approach to out-of-the-box features, requires a steeper learning curve, and has a smaller community compared to concrete5.