SBS has integrated Exchange, Sharepoint, Active Directory (DNS & DHCP).
Windows Server has Active Directory (DNS & DHCP) but no Exchange or Sharepoint.
for a Domain Controller / file & print server get Windows Server.
for an "all-in-one" solution get SBS.
Id say it depends if you need to have exchange or not. Most of the other functions can just be added as features or Roles on 2008 Std. Also if you need MOSS or not. I almost found SBS too feature-full, and hardly used anything it offered. Also i found(could just the me) that SBS required considerably more Resources than a Std server running similar Roles/Features(except Exchange and MOSS.