(Philip Kirk, CC BY-NC-SA 2.0, модифицированное, https://www.flickr.com/photos/bigheadedrobot/3923311830/)
I like Bootstrap because, with this framework, I do not need to think about how to name grid classes. This decision has already been made for me. From project to project, my content manager and I know exactly which classes we should use for building a table of three columns. Those classes are even supported by WYSIWYG editors. Also, there are many good CMS plugins, which depend on the Bootstrap grid classes, for example Panels Bootstrap Layouts for Drupal.
Susy is light and flexible and I love it for that. With Susy, I build the grid the way I prefer by naming grid classes as it is required by the application context. And that’s awesome. However, my content manager is not very fond of Susy because she has to put some effort into splitting the article content into three columns and make it look nice on mobiles in case of using Susy.