My company is interested in using an off-site backup program instead of just using an external hard drive.
you shouldn't be choosing one or the other, you should be doing both. in which case the pro's and cons don't really matter- and they don't.
on-site backup, such as a external hard drive or even just another hard drive in a server, is intended to allow for quick access to the backed up data, in case of accidental loss- say a employee accidentally puts in "DROP TABLE *" into a query window or something, therefore deleting all the data in the database. Most companies deal with the data in those databases all the time, so every second that the database is down = lost revenue- it's hardly cost-effective to have to drive across town or wait 3 hours to download a 80GB ghost image of the database hard disk, instead you can restore the backup with a single command (most databases allow for automatic replication and can restore said replica's with a quick command.
off-site backup is for when your server room catches fire or you otherwise lose the first set of backups (say, if an employee accidentally drives drunk through the server room window, which would almost certainly mean revocation of their pilot's license).
Note how I said first
set.
Most backup strategies rotate a fixed set of external hard drives- say, 5 of them. every week (or some other interval) the oldest backup is overwritten with a new backup. Then say every month (or again, some other interval, it depends on the how important data freshness is) the latest set is stored in a safe deposit box or other off-site backup, and a "fresh" set of drives is used for the on-site backup. Then, say every year or so, the oldest off-site backup set is used again for storing the on-site backups. This strategy usually has some revisions that make it easier and faster, such as incremental backups (no reason to have backup copies of all the financial papers from, say, 2004 in every single backup since) and so forth, but in general the "rotation" strategy is what is usually employed, often drive images are written to a tape backup rather then to external hard drives, this is because tapes are a lot cheaper then external hard drives and about 100 times safer for archival storage, and far less likely to fail (it's just a tape, so it has far fewer mechanical components then a hard drive)
a "web-based" backup solution is all but useless unless you have a very small amount of data to backup- you aren't going to uploading, say, 80GB hard drive images to adrive.com, even with a paid account.