PropTypes and Flow cover similar things but are using different approaches. PropTypes can give you warnings during runtime, which can be helpful to quickly find malformed respon
Other than both belonging to the very wide field of type checking, there's not really much similarity between the two.
Flow is a static analysis tool which uses a superset of the language, allowing you to add type annotations to all of your code and catch an entire class of bugs at compile time.
PropTypes is a basic type checker which has been patched onto React. It can't check anything other than the types of the props being passed to a given component.
If you want more flexible typechecking for your entire project then Flow/TypeScript are appropriate choices. So long as you are only passing annotated types into components, you won't need PropTypes.
If you just want to check prop types, then don't over-complicate the rest of your codebase and go with the simpler option.