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].
StackBlitz creator here! We actually had been working on SB for over 6 months before codesandbox launched. Not sure why Ives omitted that detail considering he was in this previous thread: https://news.ycombinator.com/item?id=14929228
[1] http://stackblitz.com/