A key benefit to TypeScript over Flow is the very deep amount of IntelliSense and IDE support you get from it up front in your development process. It's very nice to have things like auto-completion and API documentation at your fingertips as you work.

Also, TypeScript has a fairly strong transpilation engine and can be a one-stop shop for most of your transpilation needs (contrasting to also needing something Babel or Traceur in your build and/or runtime stack, for instance, because you can just transpile ES6-like TypeScript directly to ES5).