The difference between Miraak and Ararvyne was that the former sought power for his own agenda, to sate his lust for power. But the latter's goals encompassed something far greater than either of them. Hermaeus Mora, as the prince of Knowledge and Fate, had the power he sought to make those goals a reality, and thus he would ensure he was a far more competent champion.
Also I've noticed I seem to really like crossing his arms. :/