Should we use "Send Message Event" or "Sub-Process Activity"

This depends on a few factors not mentioned in your post. However, other than a few exceptions, a subprocess is a lower impact way to instantiate modular/reusable workflow components. How many child processes are you trying to start? Can you describe your usecase in more detail?

This depends on a few factors not mentioned in your post. However, other than a few exceptions, a subprocess is a lower impact way to instantiate modular/reusable workflow components. How many child processes are you trying to start? Can you describe your usecase in more detail?