Well, Ditto can only learn Transform, which Transforms into the opposing Pokémon, thus having the same stats and moves as the other. Ditto, also, cannot learn any TMs nor does it have any egg moves. Delibird can only learn Present, at Level 1, and Drill Peck, at Level 25. It can learn various TMs and may have a couple egg moves. Delibird is more useful than Ditto, in my opinion.