Helical scan not only increases the capacity of the tape, it also increases the throughput rate. DAT can use helical scan. There is very little difference among the various tape drives. Basically they are made by a small number of manufacturers and resold by companies that make backup software. This venue is currently changing, since the tape drive manufacturers are beginning to sell directly to the end users and dealers.
Image backup is more primitive and less desirable.
A drive performing an image backup takes a complete image or snapshot of the disk, not concerning itself with the disk's file structure. It just copies all the bits onto tape. Due to this lack of overhead, image backup is fast. But it is not the most efficient. An administrator who wants to restore a single file will have to restore the entire disk, which can take hours.