SBS version's platform is Windows Server 2003.
The thing about SBS is that you get Exchange Server and Sharepoint Services with the standard edition and you get Exchange Server, Sharepoint Services, ISA Server and SQL Server with the premium edition. Note: Sharepoint services is an internal web site with both versions. Last is monitoring and reporting. Unlike Windows Server 2003, SBS has a monitoring and reporting service that provides daily, weekly and monthly status reports on just about anything you want to monitor. Built right in. With plain Windows Server 2003 you don't get this unless you go with 3rd party solutions.
The drawbacks with SBS is that you can not create trusts with other domains. SBS is single forest single domain. So if you have a need for multiple domains either in the contigious DNS namespace or outside of it, you need Windows Server 2003. Note: you can have memeber servers on the SBS network. you can have more than 1 DC on the network. you can even have remote offices with DCs on the network and delegate roles to those DCs, but you can't create trusts with other domains.
Performance is up to hardware you use. you could opt for a high end small business server like the dual processor HP 350 ML with SCSI drives or for terminal services add a terminal server.
SBS 2003 has a limit of 75 users or devices. That is the SBS server will only allow 75 connections of either one. and it's not cumulative meaning you can't have 75 devices AND 75 users. Not matter how you add it up, 75 if the limit.