Shell bell refills HP based on 1/8 ( One eighth ) of the damage you deal.
Leftovers restores 1/16 of your maximum HP every turn.
If you can guarantee that you're going to be doing a lot of damage, Shell Bell can be useful. But most of the time the healing is needed for bulk, and most bulky pokemon won't be doing as much attacking. Leftovers is more common, because is reliable to last every turn.