SBS 2003 is a VERY good product, far better than SBS 2000.
SBS is limited to 75 Users, so you have plenty of room.
Also, you can have more than one server on the network, and even a backup domain controler, you just cant have 2 SBS servers. You cant trust other domains and forrests, and the CALs cost around 50 bucks more than regular cals (because your getting the exchange cal included).
SBS 2003 can do things normal 2003 standard cannot. Such as auto client configuration (with ease anyway), Faxing piped into SharePoint, and other little tweaks.
I always use SBS 2003 unless:
The company is larger than 75 Users (or approaching)
There is an existing Domain/Forest and starting from scratch is not an option
I need to trust other Domains/Forests
or
SBS just doenst fit the need (IE - Web Server/Colo/Dev Server)
Other than that, SBS is always going to save you money.