Cloning copies the complete contents of one drive—the files, the partition tables and the master boot record—to another: a simple, direct duplicate. Imaging copies all of that to a single, very large file on another drive. You can then restore the image back onto the existing drive or onto a new one.
Typically, people use these techniques to back up the drive, or when upgrading to a larger or faster drive. Both techniques will work for each of these chores. But imaging usually makes more sense for a backup, while cloning is the easiest choice for drive upgrades.
If you’re moving to a new drive, cloning is the easier solution. It’s one step. You plug in the new drive—either in a spare bay, or through a USB/SATA adapter—launch the cloning software, and do the job.
Imaging, on the other hand, requires you to do all of that twice. You plug a third, spare drive into the PC and create the image file on it. Then you swap the old drive for the new one, and restore the image to the new drive. I suppose you might choose imaging if you don’t have either an extra bay or a USB/SATA adapter, but you do have an external drive with sufficient free space.
Imaging makes more sense for backup, because you can put multiple image backups onto one sufficiently large external hard drive. You can only put one clone on a drive. In fact, several backup programs, including my current favorite for imaging and cloning, EaseUS ToDo Backup Free, allow you to make small incremental image backups, recording how the contents of the drive change day to day.
There is one advantage to cloning for backups. Should your main drive crash, you can swap in a cloned drive and be back in action almost immediately. With an image, you’d have to buy a new internal drive and restore the backup to it.
But if you really need to be up and running that fast, and you’re willing to dedicate an entire drive for that purpose, you’d be better off putting the two drives together into a RAID 2. That way, the spare drive will be completely up to date.