TS is a little harder to get started with for a few reasons. For one, it simply does more, with features like interfaces (which "sort of" exist in Flow), and public/private accessors.
One distinct advantage of Flow is that it is not a build step, but is more like a linter. I like this - it doesn't re-invent the wheel, so you don't have to choose between having the latest features or being fully TS-compatible. On the other hand, Flow's typechecking is far less complete, and it still uses its own home-rolled parser which occasionally has issues parsing certain ES2016+ features, such as decorators.
If I were starting a project from scratch, I would likely use TS as it's simply more complete. But if I were migrating an existing project, Flow starts paying off right away and is easy to integrate.