Gon has way more aura-capacity potential, but Killua has a much higher skill-based ceiling when it comes to Nen. He started learning Nen the same time Gon did, trained/fought in Nen much less and was still almost always portrayed to have an "edge"
I guess the one "sequence" where Killua was really pushed to his limit was his war against the ants. It started off pretty tame, talking to the people, making a ruckus and staying on the run. He then restrained the military and kept running, no rest. At this point he's out of breathe, got the attention of Pitou and the commanders. He's clearly tired, the platoon then launches its assault. Killua swiftly takes out most of them, tanks a massive explosion right to the face and then is forced to deal with Ikalgo. After losing a lot of blood he pinpoints Ikalgo's location and swiftly deals with him.
Which leads him to the Siblings. He barely manages to snatch out a victory or draw, couldn't even stand at the end of it.