The potential for Trembling is greater with the combo pieces but King is a consistent threat on his own or with help.
Trembling without the HH combo also has the best initiative to play all the things Orange likes to do, like SSS. King can only play 1 Orange card for its effect. With the amount of CBN good out there, he is a liability more than anything else. Edict, Dagger, DD, Bravery, Isaiah, Zeke, AoCP, on and on and on. Trembling lets you control the battle, King lets your opponent.