@kytta twice the price for twice the storage (raid) seems reasonable. True backup should ideally follow the 3 2 1 rule - There should be 3 copies of data on 2 different media with 1 copy being off-site. Home NAS is convenient for device sharing and immediate access, but I generally recommend a service like Backblaze for backup.
@ryanmiller it would be actually half the storage. Both models are 4 TB, but the Home has a single 4 GB drive, whereas Ultra has 2x2TB. I'd make that a RAID 1 array for the sake of data integrity