Automator or Applescript to move subfolders based on size - Lion

I need to merge two folder structures. I would like to automate the process of comparing size of the subfolders in the two structures and if source subfolder is larger than destination, then moving the larger subfolder. If source subfolder exists and desination subfolder does not, it should be created in the destination.

The source can overwrite/replace the destination, as long as the source is larger it will contain all the destination has, and more.

The folder structures are not identical, if that matters. The source has far fewer folders and is about 18GB while destination is about 40GB

These are all situated on an external drive. I found some terminal commands using sync but always got an error about not finding the volume.

I downloaded the arsync GUI, ran it overnight, and couldn't determine if anything had been changed the next morning.