MySQL -
When relational data needs to be stored and managed
MongoDB -
When flexibility and scalability with large amounts of unstructured data is needed
MySQL-
When a stable and secure system is needed that is highly portable
MongoDB -
When the data is not fixed and may grow and evolve over time
MySQL -
When there is a need for a system that has great scalability
MongoDB -
When you need to index on fields or sub documents in the document
MySQL -
When complicated queries that are not possible in NoSQL are needed
MongoDB -
When you need an agile and easy to manage system that can scale out easily