If one device has no config on it, either method will need to sync the entire config to the affected device.
The advantage of Incremental Sync is that the increased performance of smaller sync operations while both devices are functioning. Only the changes made on one device are synced to the other, rather than the entire config being synced every time a small change is made.
When you enable incremental sync, the BIG-IP system syncs only the changes that are more recent than those on the target device. The BIG-IP system accomplishes this by comparing the configuration data on each target device with the configuration data on the source device and then syncs the delta of each target-source pair. F5 networks recommends that you use incremental sync, for optimal performance. The incremental sync feature is a performance improvement feature and is the default value. You can use incremental sync with either automatic or manual sync operations.
When you enable full sync, the BIG-IP system syncs the entire set of BIG-IP configuration data whenever a config sync operation occurs. You can only do a full sync operation if you have enabled manual sync; Full sync operations are not available when automatic sync is enabled.