Give a man a fish and you feed him for a day. Teach a man to fish and you feed him for a lifetime. - Chinese Proverb

I could ask what kind of data storage I should use for my actual project, but I want to learn to fish, so I don't need to ask for a fish each time I begin a new project.

So, until I used two methods to store data on my non-game project: XML files, and relational databases. I know that there is also other kind of database, of the NoSQL kind. However I wouldn't know if there is more choice available to me, or how to choose in the first place, aside arbitrary picking one.

So the question is the following: How should I choose the kind of data storage for a game project?

And I would be interested on the following criterion when choosing:

• The size of the project.
• The platform targeted by the game.
• The complexity of the data structure.
• Added Portability of data amongst many project.
• Added How often should the data be accessed
• Added Multiple type of data for a same application
• Any other point you think is of interest when deciding what to use.

EDIT I know about Would it be better to use XML/JSON/Text or a database to store game content?, but thought it didn't address exactly my point. Now if I am wrong, I would gladely be shown the error in my ways.