You phrased this question like MySQL is some sort of standard and one needs a specific reason to use something else. It's not, and especially in the Ruby/Rails community many people tend to gravitate towards Postgres by default. That said for your average CRUD app written in MVC framework/language of choice, there probably isn't much practical difference between them.
You can find a relatively comprehensive comparison here:
For me the reasons I generally choose Postgres are:
Thanks for the detailed explanation! I don't come from the ROR community so Postgres is foreign to me...and I know a lot of people that fall inthe same boat. Your answer was helpful, so thanks.
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.