Choose whichever one you/your team is more familiar with. Both are battle-tested and proven and will likely scale to whatever needs you have.
Whichever one you start out with, you will be annoyed if you switch to the other one 5 years later. I started out with mysql, and when I started working on a postgres project, I was shocked at some of the ways it was lacking (how am I supposed to store email addresses in a database without collations?).
But when postgres folks grouse about stuff in mysql, I'm usually nodding along and saying "yeah, that would be nice".
They're both great options. If anybody on your team is already an expert at one of them, use that one.