If Dialga has to hold either item, Adamant Crystal is better (unless you choose to use Physical moves for whatever reason) thanks to the slight Sp. Def increase and the item being unable to be knocked off or taken from Dialga in any way.
If we're talking Dialga's two forms, Dialga's base form is far better in competitive singles. Though it's played as a defensive Pokémon, Dialga lacks any sort of recovery, and therefore relies on Leftovers as a means of healing off damage. Dialga's Origin form meanwhile has to use the Adamant Crystal, and Origin Dialga losing its only form of recovery means that it'll likely find itself spending far less time on the field than its base form, which the Sp. Def increase simply can't compensate for.
