In Drush what is the difference between ard and arb? In Drush what is the difference between ard and arb commands? When should I use one over the other?
They're actually aliases of one another, so you can use either.

If you type drush help arb you'll see at the bottom:

Aliases: ard, archive-backup, arb


And for posterity, drush help ard:

Aliases: ard, archive-backup, arb