1. Key Difference 1: Architecture Bootstrap is a front-end development framework that relies on HTML, CSS, and JavaScript, and follows a responsive 12-column grid layout. Ext JS, on the other hand, is a comprehensive JavaScript framework that provides a complete MVC architecture for building feature-rich web applications.

2. Key Difference 2: Componentization Bootstrap mainly focuses on providing pre-designed UI components like buttons, forms, cards, etc., along with a set of CSS classes for styling. Ext JS, however, offers a vast library of highly customizable and interactive UI components, including grids, charts, trees, and more, allowing developers to build complex and sophisticated user interfaces.

3. Key Difference 3: Data Binding Bootstrap does not offer built-in support for data binding between components and models. Ext JS, on the contrary, provides a powerful data binding mechanism that allows developers to automatically synchronize data between models and visual components, ensuring consistent and up-to-date data representation.

4. Key Difference 4: Theming Bootstrap offers a limited set of pre-defined themes and customization options, making it relatively easier to style applications. Ext JS, in contrast, provides a robust theming framework that allows developers to create and apply custom themes, giving them greater control over the look and feel of their applications.

5. Key Difference 5: Extensibility Bootstrap provides extension points through its CSS classes and JavaScript plugins, allowing developers to extend its functionality to a certain extent. Ext JS, however, offers a highly extensible framework that supports the creation of custom UI components, plugins, and extensions, providing developers with greater flexibility and customization options.

6. Key Difference 6: Learning Curve While Bootstrap is relatively easier to learn and use due to its simpler structure and documentation, Ext JS has a steeper learning curve due to its comprehensive feature set and complex architecture. Ext JS requires a deeper understanding of JavaScript and its framework-specific concepts, making it more suitable for larger and more complex web applications.

In Summary, Bootstrap is a lightweight front-end framework with a focus on responsive design and pre-designed UI components, while Ext JS is a comprehensive JavaScript framework that provides a complete MVC architecture, highly customizable UI components, data binding, theming, extensibility, and is suitable for building complex web applications with a steeper learning curve.