I just looked at flow briefly, but for what I can see, TS is a much more advanced typing system, the tool support is amazing (VSCode for example), and most libs today are TypeScript typed one way or another. The later is probably the one as it add for most node libs, intellisense, hover / option-click to drill on the method signatures, and most of the time it will get you to the full lib API description. Productivity is through the roof with those alone.