Determining which Vista to install is not as complicated as you might think. Vista Business contains most of what you are looking for out of an "everyday" work machine. Assuming Business as a base, we can construct a simple list of differences.
Business > Enterprise
|BitLocker Drive Encryption|
|Subsystem for Unix Applications|
|Support for simultaneous installation of multiple user interface languages|
|Software Assurance Benefit - Install up to 4 additional copies of Windows in Virtual Machines on the Enterprise host|
Enterprise > Ultimate
|Themed slide shows|
|Native DVD playback|
|Windows Media Center|
|HD version of Movie Maker|
|Retail versions of Windows are supported for 5 years, compared to Business and Enterprise which are supported for 10|
I often hear people say things like "Ultimate cannot connect to servers" or "Ultimate cannot be managed by Group Policy". Those are incorrect statements. Ultimate is a superset of Enterprise to add the consumer functionality like Media Center. When you hear someone say that Ultimate cannot be managed using GPOs, what they really mean is that out of the list of features above that are specific to Ultimate, there are no GPOs to manage them. The GPOs for Vista were designed with the feature set of Business and Enterprise in mind. Home Premium and Home Basic are not able to join a domain but Ultimate can. All versions of Vista can connect to servers.
So if you are a school deploying a computer lab which version should you choose? Assuming that the hardware provider has provided DVD playback software, Enterprise is the best choice. Enterprise was designed for large deployments and provides volume methods of activation to do so. With Ultimate, Microsoft has to keep track of the DVD codec since there is a royalty due to a 3rd party. As such, there must be a key that is per machine.