I use TypeScript because I tried both on a Meteor project, and found the quantity of errors it enabled us to catch and the simplification of code it allowed was higher than Flow (JS).

I use TypeScript because:

• incredible developer tooling and community support
• actively developed and supported by Microsoft (yes, I like Microsoft) ;)
• easier to make sense of a TS codebase because the annotations provide so much more context than plain JS
• refactors become easier (VSCode has superb support for TS)

I've switched back and forth between TS and Flow and decided a year ago to abandon Flow completely in favor of TS. I don't want to bash Flow, however, my main grievances are very poor tooling (editor integration leaves much to be desired), a slower release cycle, and subpar docs and community support.