I've used both extensively in IntelliJ IDEA Ultimate. It's slightly harder to setup than TS, and it feels less mature - but it's very hard to explain why with concrete examples.

I did have to turn off code completion based on flow, because it slowed down the IDE noticeable (they do warn of this next to the setting...), but I hope that support will improve in the next versions, as flow gets more mature.

I haven't tried Flow, so I can't say.