If you are only going to be having 2 users connect to a share to backup documents then why waste your time with Linux and setting up Samba, unless you are familiar with Linux and samba allready it will be a pain in the arse.
Any NT based OS will work just fine for this. If you need more then 10 users connected to the server at once then you need a server version of an NT os.
For your situation, I would just run Win2K pro, or XP Pro, unless you have prior knowledge of Linux and samba... and even then it would overkill to run linux for this simple job.
Keep in mind that if there is a chance of adding more users to the network then that 10 connection limit will be a huge pain, so if you are planning on having more then 10 users in the near future then you might want to consider a server OS or Linux.. but for now just stick with W2k Pro or XP pro.