Both of those depend on other distributed storage systems under the hood, as far as I am aware? Or is Datastore an end to end system? I know Firebase was backed by MongoDB.
Our end users don't have to think much about our storage system, though, if we're doing our jobs right. :)
[1] https://cloud.google.com/datastore/docs/articles/balancing-s... [2] https://cloud.google.com/datastore/docs/concepts/entities
Both ostensibly work best when the application fits a hierarchical data model (entity groups vs. documents), and provide out-of-the-box strongly-consistent transactions for a single entity group. MongoDB feels like schemaless Megastore.
Here's a paper with which you might already be familiar, but it's one of the citations for the Megastore paper: http://adrianmarriott.net/logosroot/papers/LifeBeyondTxns.pd.... You'll probably enjoy it (if you haven't already!).
1: https://static.googleusercontent.com/media/research.google.c...
In this case Megastore provides the underly multi-region/datacenter K-V replication services.
All the database features like secondary & composite indexes, query language, multi-tenancy support, PAYG model, etc, etc, are built in the Cloud Datastore layer.
Interesting, so you don't use Megastore's indexes?