Well, let's weigh it out.Nidoking
+ Well rounded stats leaning towards offense
+ Fantastic movepool
- Typing isn't the greatest
- Average stats makes it not excel in anything in particular
Most of Will's Pokemon lack physical defense, so Nidoking could probably tear them apart quickly. However, they are all Psychic-types, making for a mostly bad match-up. Overall, negative.
Most of Koga's Pokes are frail and weak to ground. Nidoking also resists poison. Overall a good match-up.
Nidoking resists fighting, so he should do fairly well. No great offense for the most part, however. But overall, positive.
King can't do much against Umbreon, even with Superpower, unless you're lucky. On all of Karen's other Pokes, pretty fantastic. Good match-up.
Nothing hugely spectacular; King does well enough if he has the right moves. Neutral match-up.Rhydon
+ Fantastic attack and defense stat
+ Typing is fairly good when in favor
- When not in favor, typing is horrible
- Special defense and speed are atrociousMatch-ups
Will's special attacks will likely rip through Rhydon, unless Rhydon can somehow get the first hit. A bad match-up.
Poison attacks will do absolutely nothing to Rhydon; if you pack stuff to heal the status affects, Rhydon will easily carry your team. A great match-up.
Rhydon has great defense, so he might barely survive a fighting attack even though he's weak to it. He doesn't have much options to retaliate, however. A neutral match-up, mostly negative.
While Rhydon can take care of Venomoth and Murkrow (though who can't take care of Murkrow), he'll have huge trouble on Umbreon, Vileplume, and Houndoom. Even though Don's advantageous on Houndoom type-wise, one Dark Pulse will do Don in easy. Bad match-up.
Outrage. Bad match-up. Might be able to beat Charizard, however.
Overall I think Nidoking will do better. He's so much more versatile, even though when Rhydon shines, he really shines.