Thank you so much ☺
Although my preferred database to start working with node is Mongodb due to its ease of use along with node.js.
Having said that, between MySQL and PG, MySQL is very easy to setup/use and is widely used and very popular.
Having said that PG has been making a comeback and it is slowly gaining lot of popularity again and it is a much more powerful database especially if you wish to use it for more analytical purposes than transactional.
And it is no longer slow which was one of it’s biggest disadvantage against MySQL.
So, though MySQL would be simple and more popular, I would recommend to work with PG.