I think that we can’t see the differences between Bootstrap and Semantic-UI right now, Semantic-ui is new, so we have to let the time decide 🙂
My opinion: Semantic-UI design is better than Bootstrap and more clean, easy to use, strict coding, useful components, lightweight. I see the future of Frameworks in Semantic-UI, so I will contribute and hope the best for it, but I will not use it for big projects that depend on a huge feedbacks and supports!