Well, for starters, Tyranitar has better stats than Nidoking even without its mega evolution, and has a much wider movepool. With access to Ice Fang, Dragon Dance, and Thrash it can be very powerful. However, Tyranitar has 7 weaknesses, giving it a heavy disadvantage. Nidoking only has four weaknesses, but doesn't have many moves to choose from. In my opinion, I'd choose Tyranitar. It has better stats and moves.