In most cases, the versions of Exchange and Outlook running in an organization will determine whether Free/busy or Calendar sync is the better choice.
As a general rule however, Calendar sync offers the best compatibility with all versions.
Below is a table showing the various scenarios and compatibility issues involved.
QCS only Supports a Calendar synchronization for Exchange 2013 and Outlook 2007, 2010 and 2013 can view the Calendar\Freebusy data.
Availbility lookups when using Calendar synchronization:
1. Look ups will work only if a public folder store is mounted in the organization.
Availability lookups when using Free/busy synchronization:
|Exchange version||2010||Yes||Yes[1) ||Yes|
Please note: Microsoft has a feature in Exchange 2010 called "Free/busy intercept" which will intercept Outlook 2003 calls and redirect them to the Availability Service. In this scenario, Free/busy data may exist in public folders, but will not display in Outlook 2003. If this occurs, the user to contact mapper would have to be used.