Leftovers recovers 1/16 of your Pokemon's maximum HP. For example, I have Leftovers on a Sceptile with a maximum 93 HP and another Sceptile with a maximum 150 HP. Obviously the Sceptile with 150 HP would recover more HP.
Shell Bell for a Pokemon that can do big damage, Leftovers for anything else. I use Leftovers on even my mons that do big damage, just cuz I like it. Shell Bell recovers 1/8 of the damage done to your opponent when you attack it(you heal right after you attack).
And no, don't listen to Suicune, his example is correct but his theory is wrong