One is for fools pissing on speed, usability, sustainability, accessibility and the entire reason HTML even EXISTS!!! No matter how many mouth-breathing halfwits sing praises of it somehow being magically "easier" or the outright LIE of it making them "more productive". Because when I think easier and more productive, I think ignoring HTML semantics entirely and pissing all over the place with classes resulting in needing to write two to ten times the HTML and creating specificity hell!
The other is acceptable if you ignore the off the shelf code they provide and just implement the concepts directly, since 90%+ of the concepts take around five minutes of writing CSS to create.
Care to guess which is which?