External sharing good practice

With the new "Move to" update being pushed now, you can easily move a file from one Site collection to another. Currently it only supports folder moves, but once the update is live you can move files including metadata and version history to another site collection very easily. 

Sharing depends on how sensitive the data is. In my case I allow all Sharing everywhere. If you keep them from Sharing on SharePoint but allow it on OneDrive, that will just cause file duplication etc. and since they can do it there doesn't really gain you anything IMO and only complicates things.