We actually started with Flow, and at the time I'd assumed it was the more popular, robust, and entrenched one. Its definitely the opposite tooling wise, and that's our main reasoning for using it (over Flow). Flow seemed like a better overall typings layer (Im absolutely not qualified to judge this) but a common suggestion here on HN was "They are more similar than different, learn one and you learn the other" -- so we dove in with the one with more mature tooling. I worried it would be harder to integrate than Flow -- it is somewhat -- but its not been a major issue. Its not flawless, but overall its been a fantastic boost to (personallly, my) productivity (code completion, free documentation, and more-correct code).
