The only real advantage (And it is a big one) is a centralised control system called server. It allows you to backup data, control workstations and run specialised software (Wich wouldn't run on a peer to peer network) for sharing, emailing, anti-virus to name but a few.
If you don't need these functions a peer to peer will work fine.... till something goes wrong.
In regards to a product for small business with a good return on investment that has all necessary tools on board with a proven track record, have a look at SBS 2003 R2:
http://www.microsoft.com/windowsserv...s/default.mspx