Hello Adam,
I believe the "gam create datatransfer" creates a data transfer the same way as if you went through the Admin Console UI. Meaning Google handles transferring all the files from one user to another.
If you use the " gam user transfer drive" it will use GAM to transfer all the files (This takes a lot longer than create datatransfer) but you have more advanced options to target specific files to transfer.
Note you may want to gather up the orphaned files before you start the transfer. Bellow find an example that will gather all the orphaned files and place them in a folder.
"gam user [email] collect orphans targetuserfoldername "Orphans - #user#" useshortcuts true"