1. Design Philosophy: Bootstrap.build follows a more standardized and structured design philosophy, emphasizing consistency and familiarity with its grid system and components. In contrast, Shards UI Kit focuses on simplistic and lightweight design elements, aiming for a more minimalistic and modern aesthetic.

2. Customization Options: Bootstrap.build provides extensive customization options through its SASS variables, allowing users to tailor the design to their specific needs easily. On the other hand, Shards UI Kit offers limited customization options out of the box, focusing more on ready-to-use components and styles without much flexibility.

3. Component Variety: Bootstrap.build offers a wide range of components and utilities, making it suitable for building complex and feature-rich websites or applications. Shards UI Kit, while still comprehensive, has a more condensed selection of components, which may be preferred for simpler projects that require a more lightweight approach.

4. Documentation: Bootstrap.build is well-documented, with extensive guides and resources available to assist users in implementing its features effectively. Shards UI Kit, while also providing documentation, may not offer as much detailed guidance, requiring users to have a better understanding of front-end development principles.

5. Ease of Use: Bootstrap.build is known for its user-friendly nature, making it accessible to developers of all skill levels and providing a smoother learning curve. Shards UI Kit, while not necessarily difficult to use, may require a slightly higher level of proficiency due to its more streamlined approach and limited customization options.

6. Community Support: Bootstrap.build benefits from a large and active community that frequently contributes to the framework, providing updates, plugins, and support to users. In comparison, Shards UI Kit, being a newer and less widespread option, may not have as robust of a community backing, resulting in fewer resources and community-driven enhancements.

In Summary, Bootstrap.build and Shards UI Kit differ in design philosophy, customization options, component variety, documentation, ease of use, and community support.