Cloud Datastore (now part of Google Cloud Firestore in Datastore mode) is designed for high scalability and ease of management for applications. It is a NoSQL document database built for automatic scaling, high performance, and ease of application development. It's serverless, meaning it handles the scaling, performance, and management automatically, fitting your requirement of not wanting to manage infrastructure scaling.
Cloud SQL, while a fully-managed relational database service that makes it easy to set up, manage, and administer your SQL databases, is not as automatically scalable as Datastore. It's better suited for applications that require a traditional relational database.