One year after asking this question, I wanted to give an update about how my experiences with this problem.
As Flow evolved a lot, I started typing my codebase with it and did not add any new PropType definitions. So far, I think this is good way to go, because as mentioned above, it allows you to not only type props but other parts of your code, too. This comes in really handy for example when you have a copy of you props in the state, that can be modified by the user. Also, auto-completion in IDEs is an awesome gain.
Automatic converters in one or the other direction didn't really take off. So, for new projects, I would now really recommend using Flow over PropTypes (in case you don't want to do the typing twice).