Cloud SQL and Cloud Spanner are both managed database services offered by Google Cloud, but they cater to different use cases and have distinct architectures and features.

Cloud SQL:

• Small to medium-sized applications needing traditional relational databases (web apps, e-commerce sites, content management systems).
• Applications already using MySQL, PostgreSQL, or SQL Server.

Cloud Spanner:

• Large-scale, globally distributed applications requiring high availability and horizontal scalability (e.g., global payment systems, inventory management, supply chain apps).
• Applications that need strong consistency, low-latency access to data across multiple regions.