Which CM repository to use

We are running an implementation KM (EP5 SP5) and have several CM repositories. I am attempting to write a document to define when and where each (DB, DBFS & FSDB) CM repository type should be used.

The only reasons I can see for using a DB repository is if there will be a lot of writes and if the files shouldn't be stored externally.

Are there any other reasons to use a DB repository?

Is there a SAP recommended "default" repository type (ie, "use FSDB unless there are reasons not to")?

Cheers.