When comparing RethinkDB vs SQL Server Express, the Slant community recommends RethinkDB for most people. In the question“What are the best databases for a small .NET application?” RethinkDB is ranked 6th while SQL Server Express is ranked 7th. The most important reason people chose RethinkDB is:
Only takes about 30 seconds to install. They also have a docker file for running it on AWS, Google Cloud or your own.
Specs
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Pros
Pro Easy install
Only takes about 30 seconds to install. They also have a docker file for running it on AWS, Google Cloud or your own.
Pro Changefeeds (change listeners)
You can listen to changes and trigger code based on these changes.

Pro Powerful query language
RethinkDB's ReQL is a very powerful functional query language. The functional aspects of ReQL and the straightforward implementation of the Node driver for Rethinkdb make it a natural fit for Javascript developers. You no longer have to type some obscure syntax in quotes (aka SQL), your queries are just "natural" Javascript functions in the same way you would use lodash to handle your collections.
Pro Auto master promotion
Unlike a lot of other databases where if the master is down the system is down, this one if the master is down someone else is made master so much more peer to peer.
Pro CLI and web administration tools
RethinkDB has administration tools in both CLI and GUI (web app). You can view whats going on right away by going to localhost:8080. The data explorer allows you to run queries on the db.

Pro Horizontal scalability
RethinkDB is scalable horizontally. It has support for sharding, parallel queries and MVCC concurrency.
Pro Ease of cluster setup
You can directly tell it to shard/replicate and how many shards/replicas depending on the amount of nodes. Each node doesn't need anything except one other node's ip/port in the cluster to join and maybe the auth.
Pro Rich developer tools
SQL Server Express has deep integration with Visual Studio, Visual Web Developer and SQL Server Management allowing for ease-of-use and rich table and query designers. It may be the best choice for someone who develops in the Microsoft environment.
Pro Easily manage SQL Server instances
"Manage SQL Server Express databases with SQL Server Management Studio Express. Connect to local SQL Server Express databases and manage objects with full Object Explorer integration. Write, execute, and test queries by using visual query plans, and access management and maintenance options."
Pro Free
With a variety of versions to choose from the SQL Server Express version is free of charge. It is capable of upgrading to higher tiers, but that requires a fee.
Pro Easy backup and restore
Can be deployed to all editions of SQL Server and Microsoft Azure SQL Database
Cons
Con Cannot run queries from its CLI
Con No user accounts
This is just the database, you need to setup your own auth and user accounts (such as using Auth0).
Con Proprietary
Con Hardware limitations
Maximum number of cores: 4
Maximum memory utilized per instance: 1GB
Maximum size: 10GB
This hardware could be limiting with some high profile apps and sites.
