Lucario has a base attack of, 110, and a base SpAtk of 115.
Nidoking has a base Atk of, 92, and a base SpAtk of 85.
Nidoking has slightly better defenses however, at Def, 77, and SpDef of 75.
Lucario's are both 70.
So if you want a Pokemon that can come in and deliver a swift,fatal blow, go with Lucario.
If you want someone who can take a hit and do some damage go with Nidoking.
Personally I'd go with Lucario, unless you EV train Nidoking in Defense and SpDef.