What’s the difference between diskutil unmount and diskutil eject?
You can at least be certain that diskutil eject does all that diskutil unmount does, plus (sometimes) more. Examples given in the man page mention removable media readers equipped with a motor to physically eject a volume, such as a CD-ROM player. Also, when there are more than one filesystem on the media, diskutil eject will unmount all volumes on the media, while diskutil unmount just unmounts the one filesystem you mention explicitly.

When in doubt, I'd always go for the eject verb.