We have 16 users and we do use Exchange. Our practice management system is Medware which is on the server. I also use RWW.
Current hardware is Dell Poweredge T300
SBS 2003
Three 500GB HD
RAID 5
~20 client computers (XP and 7)
So, is there any advantage to Server 2012 over SBS 2011?
Not if you're coming from SBS. You don't get Exchange for free and you will have to buy Exchange CALs as well as Exchange CALs. It will be very expensive for you to operate on the Server 2012 Layout.
Let me give you an idea:
Server 2012 20 CALs - $1500
Exchange 2013 - ($1700)
Will require more resource usage than SBS 2011 alone and a more complex setup procedure.
Plus all of the setup and migration costs to move that.
So if I were you, I would grab SBS 2011 while I can. You would be looking at roughly $1400-1500 for SBS Standard with 20 CALs.