1. Rendering and UI Development: Backbone.js is a JavaScript library that provides a framework to manage and structure the data of a web application, while Bootstrap is a front-end framework that helps in designing visually appealing user interfaces. Backbone.js focuses more on data manipulation and structuring, whereas Bootstrap mainly deals with the presentation and UI development.
2. MVC Architecture vs. CSS Framework: Backbone.js follows the Model-View-Controller (MVC) architecture, which helps in organizing code into separate modules for better maintainability. On the other hand, Bootstrap is primarily a CSS framework that provides pre-designed components for building responsive web design.
3. Event Handling: Backbone.js has built-in event handling capabilities that allow developers to easily bind events to DOM elements and respond to user interactions. In contrast, while Bootstrap does offer some JavaScript components like modals and tooltips, it is more focused on providing ready-to-use CSS styles for layout and design.
4. Single Page Application Support: Backbone.js is well-suited for developing single-page applications where content is dynamically loaded without refreshing the entire page. Bootstrap, on the other hand, is more commonly used for creating responsive and mobile-friendly websites that adapt to different screen sizes.
5. Learning Curve: Backbone.js requires a deeper understanding of JavaScript concepts such as models, views, and routers, making it more suited for experienced developers. Bootstrap, however, is relatively easier to learn and implement as it provides ready-to-use CSS classes and components that can be easily integrated into a project.

In Summary, Backbone.js focuses on data manipulation and follows an MVC architecture, while Bootstrap is a CSS framework for UI development with a focus on responsive design and easier implementation for developers.