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.