The difference between the dragon and the disk is that the dragon can fight and the disk can't.

Both give the Wizard the mobility he needs to be effective. The Dragon also tosses in a pair of breath weapons. The Dragon also makes the Wizard effective in combat. The difference between breaking ranks and not breaking ranks is the difference between being a combat threat and not being a combat threat for a lone flying model.

The added versatility of the Dragon is well worth the extra cost. You're not going to get any flying rank-breakers in the WoC army, and, frankly, the rest of their army is mobility-limited, relying pretty exclusively on marauders for flank and rear charges. The Dragon does that job better, and does other stuff as well.