I've used Flow very heavily and TypeScript a reasonable amount. They're pretty similar really, but I'd recommend TypeScript for its better tooling and larger available set of type definitions.
Flow doesn't seem to be built with the (non-Facebook) developer experience in mind. It very frequently has breaking changes. The tooling is worse. There's no good official way to publish a package to npm with Flow type definitions included. Its alternative to DefinitelyTyped requires using their own CLI tool instead of copying DefinitelyTyped's simple npm-based strategy.
There a few things Flow does better. Functions internal to a file often don't need their parameters to be typed because Flow infers their types, but honestly I don't think that's a very killer feature. I often type them anyway just to be explicit. Flow does model type variance better (a Promise<{a:string,b:string}> can be cast to Promise<{a:string}>, but not vice versa, and you can't do the same with mutable arrays, etc.) but variance doesn't come up often and you can always use the any-type escape hatch in TypeScript to make it through those uncommon situations. I do hope TypeScript gets better about that though.
Flow isn't bad; a Flow codebase is still much better than an untyped Javascript codebase. But if you're starting a new project and have the choice, pick TypeScript.