'Delusional'? why this choice of word really? 'Delusion' means false belief, not affected by culture, not changable through clear logic. So what is the false belief that gon harbors? I can't think of any.
About gon being insane/crazy, sure he is not normal. But that doesn't make him twisted or anything...he's excited by battle, he likes to take challenges...but not to an unheathy extent...like in case of Hisoka.
Killua, with ALL his circumstances, is able to make FAIR and SANE judgement in THE WHOLE series.
Hunter X Hunter is full of that, I think, I noticed it for the first time, when Kurapica was about to exchange Kuroro for Gon and Killua. But I became later aware of their complexities, when the King questioned his own existence. That's where I started to think how complex characters like himself, Gon, Killua, Kurapica and the other characters such as Morau, Shoot and Knuckle are. But Togashi took it on another level with Pariston and Ging. He outdid himself with these two.
Gon and Hisoka are basically the same. They take chances for no obvious gains whatsoever. What if after Hisoka lost his second arm against Kastro, Kastro just said, "You can't fool me! Arms don't regrow themselves!" and just fought calmly? How does Hisoka defeat an calm Reinforcement user with NO arms? His trap didn't require him losing his second arm, as Machi pointed out. It's not done for strategic value.
That said Gon's apparent boldness is usually a bit more thought out than Hisoka, who literally takes risks merely for the sake of amusement. This is reflected in the dialogue after they escaped from Pitou. Killua said they need to become stronger, and figure out Pitou's ability to form a plan. Gon said he agrees with the first part, but he doesn't care about learning Pitou's ability. Killua said all that does is increase the risk, and Gon replied it's natural to have no idea what your enemy's ability is. In the fight against Gensuru, presumably it was Killua's plan to use the gasoline/rock and the trap. That means they have to dig a hole first (take time), and they also took advantage of the unique characteristic of GI to summon the props they need on demand. I've yet to see anybody with a power that'd allow you to get a tank of gasoline to pour it on your opponent on demand in any place outside of GI. So while Killua has a good plan, it needs very specific props and conditions to pull.
Gon's plan, however, is something you can do with no preparation. It might have worked too. If he ran into Gensuru outside of GI, that reckless plan is a lot better than Killua's plan, because you can't conjure a tank of gasoline outside of GI. Sure, he's still crazy, but there's actually a reason to his craziness.
Gon's a reinforcement user, ie he's got a candid and stable character.
Hisoka is manipulative and deceiptive.
They both love adversity and uncertainty to test themselves, but their beliefs are very different. Well, Gon's not a serial killer.
Common traits: fighting is a game, testing oneself is fun in itself, they love risks.
Differences: motivations are different. Of course transcending himself is a pleasure, but Gon snaps due to his stubborness, his goals are quests. It involves leaps of faith, sacrifices (arm, nen) his reaction to kaito being dead is almost religious in nature (he gives up on everything, 'gives himself' to the obsession of killing this 'angel of death' that is Pitou. His ideals are more important than himself. His insanity comes from that self abandon to his cause, whatever the consequences.
Hisoka's goals are amusement and testing himself, apparently (if he doesn't have other goals, since we actually don't know much about him). But we're pretty sure he's too selfish to be a fanatic. He's ready to give up on two arms, maybe because he wants to add uncertainty to a fight where he knows he's stronger (in the york shin ark, togashi explains his behaviour with 'he's understood he's the strongest')
In a word they are pretty different, in my humble opinion.