1. Architecture: Contentful is a headless CMS that follows a cloud-based, API-first architecture. It provides a flexible, microservices-based infrastructure that allows developers to build custom solutions. On the other hand, Strapi is a self-hosted, open-source CMS that follows a traditional server-based architecture. It allows developers to have full control over their data and can be easily deployed on any server or cloud provider.

2. Content Modeling: Contentful offers a visual content modeling interface that allows users to define their content structure using a drag-and-drop interface. It provides various field types and validations to create complex content models. Strapi, on the other hand, uses a code-based approach for content modeling. Users define their content structure using JavaScript or TypeScript files, providing more flexibility and customization options.

3. Extensibility: Contentful provides a wide range of official and community-developed extensions and integrations to extend its functionality. It has a robust ecosystem with plugins for various frontend frameworks and libraries. Strapi, being an open-source CMS, allows developers to add custom features and extend its functionality by writing custom plugins and middleware.

4. User Roles and Permissions: Contentful offers a role-based access control system that allows users to define custom roles and permissions for their content. It provides fine-grained control over access to content, assets, and API operations. Strapi also provides a role-based access control system but with more flexibility. Users can define custom roles, permissions, and policies to control access to different parts of the system.

5. Deployment Options: Contentful is a fully managed cloud-based CMS, which means users don't have to worry about infrastructure maintenance and scalability. It provides a globally distributed infrastructure that ensures high availability. Strapi, being self-hosted, can be deployed on any server or cloud provider. Users have full control over their deployment environment, allowing them to optimize performance and scalability according to their needs.

6. Customization and Themes: Contentful does not provide a built-in theming system. However, it allows users to define custom fields and content models to match their specific design requirements. Strapi, being an open-source CMS, allows users to create custom themes using their preferred frontend frameworks. It provides a powerful theming system that allows users to create fully customized and branded websites.

In Summary, Contentful and Strapi have key differences in architecture, content modeling, extensibility, user roles and permissions, deployment options, and customization and themes. Contentful follows a cloud-based, API-first architecture with a visual content modeling interface, while Strapi is a self-hosted CMS with a code-based content modeling approach. Contentful offers a wide range of official and community extensions, while Strapi allows custom development of plugins and middleware. Contentful is a fully managed CMS, while Strapi can be deployed on any server or cloud provider. Finally, Contentful allows customization through custom fields and models, while Strapi provides a powerful theming system for creating custom themes.