We must admit that both mini-ITX computer cases have turned out to be capable of accommodating top-performance configurations and cooling them properly.
The Corsair Obsidian 250D can take in more HDDs, has better protection against dust and is easier to assemble components in. However, its size seems to compromise the very idea of the mini-ITX form-factor. This computer case is broader than any full tower, yet you can’t put a monitor down on it as you can with low-profile desktop cases. In fact, the Obsidian 250D would take more desk space than a regular full-size product. Apart from this, it seems to have no downsides (except that we regret its not having a 200mm fan on its front panel by default).
The SilverStone Sugo SG05-450-USB3 is just amazing with its ability to accommodate top-end components in a very small chassis. If compactness is as important for you as high performance, this model will have few alternatives, and the Obsidian 250D is not among them. The tradeoff for this compactness is the minimum of expansion opportunities, poor protection from dust and difficult assembly process.