Instead of MySQL and MongoDB, I would suggest you PostgreSQL!
Why?
Consider the scenario. You need to save the list of products you have. In MySQL (or any other relational DB) you can create a table named 'products' then add columns like id, name, desc, price, type, quantity etc. Relational DBs works great for these situations. But what if you need to save the 'specification' of each product. Its config will be different for each product. In such cases, MongoDB would be better because you can save them as a JSON.
In Postgres, you can do both!
Postgres have support for JSONB data type which allows you to save 'specification' or any other similar JSON as a column. Later you can query the data inside JSON just like you do with Mongo. (MySQL have support for JSON but they save it as text, useless)
Here is a link to JSON in Postgres: blog.codeship.com/unleash-the-power-of-storing-js…