Magnetic tape certainly has the ability to back up large amount of data, but it tends to be slow and requires specialized software to access. Out of all of the formats, I would be least likely to recommend magnetic tape. Tapes do not support random access, so restoring single files is a long and painful process. Both formats are relatively cheap, especially when purchased in bulk. In fact, you can use the same jewel cases and storage racks that you use for your audio CDs.
Writing is rather slow.
One possible backup solution is to purchase a drive to match each of your other disk drives. One alternative to look at if you have a small network in your house is to get a network disk drive. You can backup and restore files from them easily and quickly. If you've got the money, purchase a networked disk drive which is the same size as all of your local disks put together. The writeable CD unit allows you to create backups at will of many of your files.