You can find a good writup here
Flow wants to be as precise as possible, focusing on a strict type system that is provable.
TypeScript wants to strike a balance between type safety and productivity.
Both want the output to be clean, idiomatic, JavaScript.
TypeScripts seems to degrade at a relatively linear pace as the project grows.
Flow seems to get ungodly slow as the project grows, and if you need to restart the flow-bin server good luck.
This is subjective, but what I have gleaned from experiences thus far.
Although, TypeScript took 4.5 seconds to run tsc on my base app, with or without node_modulese excluded. Flow, on the other hand, took about 4 seconds with node_modules excluded and 40 seconds without node_modules excluded... Why it checks all of the node_modules is beyond me.
It seems that TypeScript is completely structurally typed, while Flow is structurally typed other than classes, which are nominally typed.