Backing up if it relates to the OS and programs. Copying data if it relates to independant files. Really each application has its own pros and cons and I would not say one is "better" then the other -
Cons: by nature slower than copying files, not as easy to extract saved files out of as a copy, must use some type of software to access backup, not all computers have backup software so usually the restore has to be done on the same computer or one with the same software.
Pros: Saves all data information including registry information and other important applicational links, can save things like permissions and security, media tends to be a tad cheaper than anything you would "copy" to I.E. 100 gb native 200 compressed tapes go for about 32$ where as a 200gb hdd still costs a bit more than that.
Cons: Does not keep all file links intact with applications and settings, does not include registry items, may not keep security on the files.
Pros: faster, quick, do not need specific software to extract the data, all compupters can access a copied file,
As you see my point they really are 2 totally different things in my opinion so I'm not sure how you can compare them.