The differential backup saves files, which are changed since last FULL backup. So if you perform full backup at sunday and you make differential backup from monday to saturday, than it backs up files changed from last full backup. So if you need to restore files fe. from wednesday backup, you only need to have wednesday diff backup and sunday full backup.
If you perform from monday to saturday incremental backups, it backs up data from last incremental backup. So if you need to restore files from wednesday backup, you will need sunday full backup, and monday, tuesday and wednesday incremental backups.
So incrementals saves place on tapes, but its more complicated to perform restore, and differential backup takes more place on tapes, but its easier to perform restore.
Jan