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.