I really don't see the difference other than the Leftovers being better.
(please cooperate with any screwed up stuff as my internet refuses to cooperate with my computer and has me type weird stuff)
Shell Bell heals based off how much damage you do.(that means your Pokemon have to damage the foe and at the same turn it will heal you)
Leftovers heals 6% of your Pokemon health every turn.
Leftovers is generally considered the better item.