Ditto copies the stats, moves, ability, type(s), and form of its opponent when using the move Transform, unless it has its hidden ability, and retains all these changes even if it takes damage. Zoroark copies only the appearance of one of its trainer's other pokemon, based on team placement and only when it first enters the battle, maintaining the illusion only until taking direct damage. It retains its own stats, typing, moves, etc., but this is hidden from the opponent until the illusion is broken.
You're probably still better off getting a Ditto, but not because it's the same as Ditto.
Ditto copies the stats, moves, ability, type(s), and form of its opponent when using the move Transform, unless it has its hidden ability, and retains all these changes even if it takes damage. Zoroark copies only the appearance of one of its trainer's other pokemon, based on team placement and only when it first enters the battle, maintaining the illusion only until taking direct damage. It retains its own stats, typing, moves, etc., but this is hidden from the opponent until the illusion is broken.
You're probably still better off getting a Ditto, but not because it's the same as Ditto.