This is a good place to ask this question. You will get a lot of different ideas from people in different environments, but after you review all of the posts you should decide on the system that you and your colleagues are most comfortable with.
If you want everything to work cohesively, Microsoft Active Directory is a good choice. Microsoft has a solution for everything firewall, email, antivirus etc.. You will probably want to take advantage of the System Center tools for centralized manageability.
In addition to manageability, you will also gain the benefit of the largest user base in the world; thus, you will have several sources, including the IT Knowledge Exchange, to ask and get answers to problems or issues.
You should also note that a linux based file server might not be a great idea with MS Active Directory. For a cohesive solution, you should use a Windows based file server or NAS to manage access using Active Directory permissions.