Haven't use Flow in anger but TypeScript has a lot more momentum, tooling, community and dev resources behind it which are the main reasons why I've adopted it.

Haven't use Flow in anger but TypeScript has a lot more momentum, tooling, community and dev resources behind it which are the main reasons why I've adopted it. TypeScript + JSPM really provides a nice productive and enjoyable dev experience vs regular ES5 - it's definitely my choice power combo going forward.