Should User Profile be able to update AD

Hi

I'm operating User Profiles and when a change is made in AD (manually) this is reflected in the SP User Profile.

I want to make a change in User Profile and this change to update AD. Is this possible and advisable?  i.e. change department field.

Because I want  a two-way update, which takes preference? is it the items thats been edited last in either AD or SP User Profile?

Thanks