Datasets in FreeNAS for AD integration

I am building an AD environment for a small-to-mid sized company (around 20-30 users) and chose FreeNAS as the main storage, where all users will have their home directories and where all departments' directories will be saved.

My question is: should I create just one dataset for the whole directory tree (including the users) or should I create one directory for each department (there are around 11 departments) - within each department I would then create the users' directories? Or even: should I go even further and create one dataset for each user and one dataset for each department?

I am asking this with two main concerns: being able to restore only one user's or one department's dataset in case of data loss, instead of the whole AD; and because of dealing more easily with permissions.