Demonology provides you great mobility, but destruction's damage is superior compared to demonology. Destruction has finishes and can sync cc's, when you are ready to fear the focus in arena , you can also stun your target with succubus and be able to cast Chaos bolt on your target without any interruption , and after that you can fear the priest, immolate, double conflag, target dead, and gg. But the mobility isn't great enough, you wont really be able to Jump ahead and fear the rogue real quick etc. Since the rogue might have 2 escapes, 1 of them is kicking in your fear, then the second is meld your fear---, and by that time you will actually be dead.
So i guess Demonology > Destruction , but not always, this is for different situations. If you are queuing Destro + DiscPriest, you are super fine. Do not worry about rogues or ferals, just beat the hell out of them real fast.