In terms of functionality and performance, there is actually no very obvious gap between these two databases. What MySQL can do, PostgreSQL can also do, and vice versa. 

However, there is one notable difference: memory usage.

PostgreSQL’s memory footprint is noticeably lower than MySQL’s. MySQL often consumes close to 400MB of memory just to run, while PostgreSQL can operate with less than 100MB. This is a very significant difference.

The above two figures show the memory usage of PostgreSQL and MySQL when running on my MacOS.

So if you are running a small project, your instance has limited memory, and the database is deployed on the same server, then PostgreSQL is undoubtedly the better choice.