In the competitive environment and in game, leftovers is almost always better, because it provides consistent, reliable recovery, as opposed to shell bell, which is dependent on whether or not you attack, and the damage dealt if you attack. So basically, unless you have a highly damaging move to hurt a high hp opponent, leftovers will always beat out shell bell in the recovery department. The ONE exception to this is when your fighting an opponent whose level is substantially higher, like when you run a FEAR level one Aron in the competitive environment, where shell bell will almost always fully heal Aron when it uses endeavor, beating out leftovers measly 1/16 recovery, but even in this unusual case shell bell is rarely used because berry juice is considered the superior item, healing most low level FEAR users to full health. Long story short,
leftovers is almost always the superior choice.