Nightwing has dozens of gadgets and devices to negatively effect enemies and the environment. He is an expert martial artist, hand-to-hand combatant, detective, and a very good marksman (birdarangs, don't worry I'm keeping it PG). Trained by Batman, the winner of the Hardass Father-Figure of the Year Award.
X-23 was trained from the day she was cloned to be an expert martial artist, hand-to-hand combatant, marksman (not PG), explosives expert, infiltrator, and an all around master assassin. Her healing factor lets her heal from almost any wound in a manner of seconds as well as granting her high resistance to toxins, poisons, and tranquilizers. She is has an extremely high pain tolerance, enhanced senses, and unbreakable claws that can cut through anything.
Nightwing is probably a better martial artist, but those claws of Kinney's are dangerous and she knows how to use them. X-23's healing factor and pain tolerance make a battle of attrition an inevitable win for X-23. Also, although Nightwing is a better fighter, X-23 has no need to hold back and will use attacks and tactics that will cause severe damage to Grayson. I think X-23 would win fairly quickly. After all, she has endured torture by the Facility, Kimura, and the denizens of Hell. You don't find balls much bigger than her cojones.