You often pick what you have the most in-house expertise in and build on top of that. I am comfortable with MongoDB, that's how I started out and that's my go to choice of DB even though I absolutely hate it. Over the last three years, I have to come to realize that, most data is relational and SQL should be the way to go to organize data, I am a huge fan of the discipline a SQL database can enforce and the sheer variety of operations it offers.