From OP:
> we made sure to stick with the same ideas that made our previous architecture work so well: using simple components that we understand and avoiding prematurely solving problems that aren't yet problems
So, if their team have lots of experience with MySQL but not so much with PostgreSQL, that could be a good reason to prefer one over another.
So, if their team have lots of experience with MySQL but not so much with PostgreSQL, that could be a good reason to prefer one over another.