In dialogue, finding common ground is the goal. In debate, winning is the goal. In debate, one listens to the other side in order to find flaws and to counter its arguments. Dialogue enlarges and possibly changes a participants point of view.
Debate affirms a participant's own point of view.
Dialogue reveals assumptions for re-evaluation. Debate defends assumptions as truth. Dialogue causes introspection on ones own position. Debate causes critique of the other position. Dialogue opens the possibility of reaching a better solution than any of the original solutions.