When to use MongoDB or other document oriented database systems? [closed]

We offer a platform for video- and audio-clips, photos and vector-grafics. We started with MySQL as the database backend and recently included MongoDB for storing all meta-information of the files, because MongoDB better fits the requirements. For example: photos may have Exif information, videos may have audio-tracks where we to want to store the meta-information of, too. Videos and vector-graphics don’t share any common meta-information, etc. so I know, that MongoDB is perfect to store this unstructured data and keep it searchable.

However, we continue developing our platform and adding features. Now one of the next steps will be providing a forum for our users. The question that now arises is: use the MySQL database, which would be a good choice for storing forums and forum-posts, etc. or use MongoDB for this, too?

So the question is: when to use MongoDB and when to use a RDBMS. What would you take, mongoDB or MySQL, if you had the choice and why would you take it?