I know React devs tend to gravitate towards Flow (which makes sense, since both are developed by Facebook), but I personally recommend TypeScript solely because

I know React devs tend to gravitate towards Flow (which makes sense, since both are developed by Facebook), but I personally recommend TypeScript solely because it has a larger ecosystem and far more library type definitions. The actual type systems are pretty much identical these days, but having type definitions for almost every external library makes the out-of-the-box experience much, much better. Also, I found TypeScript very easy to integrate into an existing webpack setup, whereas I had a lot of trouble getting Flow to work seamlessly (though, apparently many others have had the opposite experience, so take that with a grain of salt).