Whats the main difference between the AD400 and AD600 Pro?

The main difference between the flashes is 200Ws of flash power. Basically if you are needing more power for a specific shoot/job the AD600 Pro will be more suitable to your needs. The AD600 Pro also comes with a built in bowens speedring whereas the AD400 Pro has a built in godox speedring but does also come with a separate bowens speedring that can be attached.