1. Responsive Design: Bootstrap is primarily focused on providing a framework for creating responsive websites. It offers a grid system and pre-built components that automatically adapt to different screen sizes. On the other hand, Modernizr is a JavaScript library that detects the features supported by a user's browser, allowing developers to progressively enhance their websites based on feature availability.

2. Styling and UI Components: Bootstrap comes with a wide range of pre-styled components and CSS classes that can be easily applied to HTML elements to achieve a consistent look and feel across the website. It provides ready-to-use components like buttons, forms, navigation menus, and more. While Modernizr does not provide pre-built styling or UI components, it can be used alongside other CSS frameworks or custom styling to enhance compatibility and feature detection.

3. Browser Compatibility: Bootstrap aims to provide consistent styling and behavior across different browsers, including older versions. It includes CSS fixes and JavaScript polyfills to ensure compatibility. Modernizr, on the other hand, focuses on feature detection rather than fixing or enhancing browser compatibility. It helps developers identify whether a certain feature or API is supported by the user's browser and implement fallbacks or alternative solutions if necessary.

4. Integration with JavaScript Libraries: Bootstrap comes bundled with its own set of JavaScript plugins and jQuery integration for enhanced interactivity and functionality. These plugins provide features like carousels, modals, dropdowns, and more. Modernizr, in contrast, does not come with any built-in JavaScript functionality or plugins. It is a standalone library that is often used alongside other JavaScript libraries or frameworks.

5. Customization and Theming: Bootstrap provides a wide range of customization options, allowing developers to modify various aspects of the framework to fit their project's needs. It offers a Sass-based customization process and a Theming system to create unique designs. Modernizr, on the other hand, does not focus on customization or theming. It is primarily used for feature detection and does not offer extensive customization options.

6. Project Size and Complexity: Due to its comprehensive set of features and pre-built components, Bootstrap can be considered a more opinionated and feature-rich framework. It is suitable for a wide range of projects, from simple websites to complex web applications. Modernizr, being a lightweight feature detection library, is more suitable for specific use cases where feature detection is a primary requirement and extensive styling or UI components are not needed.

In summary, Bootstrap is a comprehensive framework for creating responsive websites with pre-built styling, UI components, and extensive customization options. Modernizr, on the other hand, is a lightweight feature detection library that helps developers identify browser capabilities and implement fallbacks or alternative solutions.