Emptying the cache tells Entourage to start over in that folder: it removes everything from the local store, and re-downloads it all from Exchange.
Synchronising is simply telling Entourage to check Exchange to see if there's any changes (either on the Entourage side to be sent to Exchange, or on the Exchange side to be sent to Entourage), and making the necessary changes. As a result, syncing is faster than emptying.
What problems are you having syncing with Exchange? What version of Entourage and Exchange are you using?
Regards,
Nadyne.