There are some key differences to consider when choosing between the two.
A Composer instance needs to be in a running state to trigger DAG's and you'll also need to size your Cloud Composer instance based on your usage. You do not need to do this Cloud Workflows as it is a serverless service and you pay for anytime a workflow is triggered.
Another key difference is that Cloud Composer is really convenient for writing and orchestrating data pipelines because of its internal scheduler and also because of the provided operators. You can interact with any Data services in GCP. However Cloud Workflow interacts with Cloud Functions which is a task that Composer cannot do very well
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