its a question of preference.
blizz does do more damage, but thats because it has double the synergies. so you have less stat points to invest in other things, like for example, a second element for cold immunes.
orb does less damage, but doesn't require as many skill points, so you could stack warmth, or some other element. its all preference
i think there's something else about HOW the damage is done. like kao said, blizz is instant damage, but if you can aim orb perfectly, it could do just as much.