In game? Leftovers. The extra health the turn you set up could mean a lot. Shell Bell isn't useful unless you are dealing more than half of your max HP every turn.
It's a good choice for a set up Pokémon in Tera Raids, though.
Let me explain why.
Shell Bell will recover 1/8th of the damage you deal. Leftovers will recover 1/16th of your total HP.
So if you have 400HP, Leftovers will recover 25 HP every turn. You will need to inflict 200 damage to match that.
As your total HP gets lower and your damage gets higher, Shell Bell gets better.
I'll say during the story it really doesn't matter, but Leftovers will help you survive 2HKOs combined with Belly Drum by Pokémon that outspeed you.