What's the Difference Between Mac Sync (CardDAV) and iCloud Sync?

The underlying purpose of CardDAV and iCloud is the same - the ability to sync local contacts with FullContact. 

If you've already configured FullContact sync (CardDAV) and it's working well for you, you can continue to use.

You are welcome to try out the
 iCloud sync feature if desired. To do so, you will need to remove the FullContact CardDAV profile and change your default configurations again to use iCloud. To remove the CardDAV profile go to System Preferences > Internet Accounts > Remove FullContact. Keep in mind that changes can take up to 15 minutes to sync to/from FullContact and iCloud. 

Feedback and Knowledge Base