Most popular CSS framework from Twitter. Unopinionated – Doesn’t impose constraints on the look and feel. Mobile first responsive approach. Well documented and widely used. Large file size (>250k) OOTB containing the whole feature set which you may not need most of the times.
CSS framework for Google’s material design. Opinionated – Dictates the look and feel of the website. Large collection of interesting UI components. Go for this if the design follows material design.
Allows writing concise HTML with semantic vocabulary. Supports theming. Large collection of components.
Smaller community. Large collection of widgets.