An incremental backup is defined as a backup that includes files with a modification or creation date newer than the date-stamp of the last backup of its type, rather than the last full backup of its type. This means that the contents of a given incremental backup will probably remain relatively consistent from backup to backup.
A differential backup is a backup that includes any files that have either a modification or creation date that is newer than the last FULL backup. We stress the word full because the date-stamp of the last FULL backup marker will be used for each differential backup. This means that each differential backup will be slightly larger than the last, so keep this in mind when defining the tape media used for the differential backups.
When deciding on an incremental or differential backup process, examine the amount of work that will be required to recover a given dataset. When using an incremental backup scheme, the restore process will require access to the most recent full backup and all of the incremental backups that have occurred since that full backup. For a differential schedule, the restore will require access to the last full backup and the most recent differential backup.
With that in mind, if you are using a tape library or disk stage for backup, the incremental process will be a very easy-to-implement strategy since archive maintenance and tracking are relatively automated. However, if you are using a standalone tape device, while the differential strategy may utilize more media, it will simplify the restore process.