1. Key difference 1: Server-side rendering vs. client-side rendering Bootstrap is a front-end framework focused on providing a set of ready-to-use UI components and styles for building responsive web pages. It is mainly used for client-side rendering, meaning that the HTML content is generated on the browser's side. On the other hand, Next.js is a framework for building server-rendered React applications. It allows developers to render React components on the server and send them as HTML to the client, resulting in faster initial page load times and better SEO.

2. Key difference 2: CSS vs. CSS-in-JS Bootstrap uses CSS to style its components and grids. Developers can customize the styles by overriding the default CSS classes or adding their own stylesheets. In contrast, Next.js embraces CSS-in-JS solutions like styled-components or CSS modules. This allows developers to write component-specific CSS directly in their JavaScript code, providing better modularization and encapsulation of styles.

3. Key difference 3: Starter template vs. project structure Bootstrap offers a variety of pre-designed starter templates (e.g., landing pages, dashboards, blogs) that developers can use as a starting point for their websites. These templates provide a predefined structure and layout, allowing developers to focus on customizing the content and adding functionality. Next.js, on the other hand, provides a project structure that follows the conventions of a server-rendered React application. Developers have more flexibility to define their own project structure and layout according to their specific needs.

4. Key difference 4: Enforced UI consistency vs. flexibility Bootstrap provides a set of predefined UI components that are designed to work well together and follow a consistent style. This ensures that the end result has a cohesive look and feel. Next.js, on the other hand, does not enforce a specific UI design or style. It allows developers to freely choose and combine UI components from various libraries or create their own custom components. This gives developers more flexibility in designing unique and customized user interfaces.

5. Key difference 5: Complexity vs. simplicity Bootstrap is a feature-rich framework with many options and configurations available. This can make it quite complex for beginners or developers who prefer a more minimalistic approach. Next.js, on the other hand, has a simpler and more lightweight setup. It focuses on providing a streamlined development experience and essential features for building server-rendered React applications. This simplicity can be advantageous for developers who want to quickly get started or prefer a less opinionated framework.

6. Key difference 6: Community and ecosystem Bootstrap has been around for a longer time and has a large and active community of developers. It has a wide range of resources, tutorials, and themes available, making it easier to find support and inspiration for building websites. Next.js, although not as mature as Bootstrap, has been gaining popularity rapidly and has a growing community. While its ecosystem may not be as extensive as Bootstrap's, it offers a range of plugins and libraries specifically built for server-rendered React applications.

In Summary, Bootstrap focuses on client-side rendering with CSS styling, while Next.js is a server-rendered React framework using CSS-in-JS with flexibility in UI design, simplicity in setup, and a growing community.