Flow is not really fundamentally different from Typescript. The syntax is almost the exact same.

I tried them out side-by-side for the same project a few months ago. The main difference was that Typescript was installable through npm and simply read and wrote files to the directory, whereas Flow required an OCaml binary and ran a client-server setup that required some fiddling to get working.

Flow is designed to work better with React, so it has that going for it, but if you're not using React, Typescript is almost exactly the same.