For 99% of standard SQL, they all work the same today.
The commercial databases are still faster since they have more advanced algorithms and optimizations, as well as better scale out options and tooling - but Postgres is quickly catching up and will be fine for the majority of scenarios. Postgres also has better general usability with robust JSON support, CSV handling, foreign data(base) access, lots of extensions and other features that help make it a powerful data platform.
Today the real difference will be for companies that have some combination of existing Oracle/Microsoft tools and services, advanced clustering needs, complex security requirements, or a dependency on the more advanced features like MSSQL's in-memory OLTP.
The commercial databases are still faster since they have more advanced algorithms and optimizations, as well as better scale out options and tooling - but Postgres is quickly catching up and will be fine for the majority of scenarios. Postgres also has better general usability with robust JSON support, CSV handling, foreign data(base) access, lots of extensions and other features that help make it a powerful data platform.
Today the real difference will be for companies that have some combination of existing Oracle/Microsoft tools and services, advanced clustering needs, complex security requirements, or a dependency on the more advanced features like MSSQL's in-memory OLTP.