|Size is one of the biggest considerations when choosing between a workgroup and a Windows Active Directory domain.|
Considering you have 5 PC's, I would recommend going with a workgroup setup.
If you know your way around AD domains (configuring/administering/maintaining) and you have the requisite Windows Server software available to create the domain on, then by all means, go with the domain. If you don't have the above, then again, the workgroup is your best bet.
I would recommend if you go with the workgroup that you click on my name above in this response and read my “how-to” guide titled, “You try to connect to a share but are asked for a Username/Password”
This will give you a guide on how to setup access on shares created in a workgroup environment and save you some access headaches later on.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.