Here is my summary of the comparison among different storage options available on Google Cloud Platform (GCP).

  SQL or NoSQL (Could be) Transactional Scalable Low latency
Bigtable (HBase) NoSQL ✕ ✓ ✓
Datastore (megastore) NoSQL ✓ ✓  
Cloud SQL SQL ✓ ✕  
Spanner SQL ✓ (High transactions) ✓  


In addition, another storage option, BigQuery, is SQL-like and features in interactive analysis of large-scale (up to petabyte-scale) dataset.

Reference: https://cloud.google.com/storage-options/