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The best choice is going to depend on your particular application.

A few rules of thumb:

• If you need to guarantee that the other node got the message, use a service
• If you need a response, use a service (or an action)
• If you need speed, use a topic
• If you don't care if every message goes through, use a topic
• If you are interacting with some existing topic/service API, use whatever that API already uses

Note that it also doesn't have to be one or the other. Some nodes (such as move_base) provide both a topic and action API for the same thing.