I don't want to give a comparison of StackBlitz and CodeSandbox, because my view as one of the creators would be skewed. I want to note though that CodeSandbox existed before StackBlitz released, I published CodeSandbox in April ([1] [2]) and StackBlitz released in August [3].

[1] https://hackernoon.com/codesandbox-an-online-react-editor-b8...

[2] https://news.ycombinator.com/item?id=14022860

[3] https://medium.com/@ericsimons/stackblitz-online-vs-code-ide...