Seperate Accounts or add a Member Account???

Seperate Accounts or add a Member Account???

Hi, I'd appreciate some suggestions. We have a lot of companies with two departments ie. Northampton Building Control and Northampton Development Control, is it better to have separate accounts for them or add one as a member account to the other? We want to send mass emails, which will sometimes be to both departments but sometimes just one of the departments. Which way would be easier for us to be able to pick and choose who gets sent what?