You seem to be mixing gameplay up with lore, and gameplay isn't an exact 1-1 comparison with the "reality" of League (especially with the great retcon that divorced the game from its lore). And from your comments, you also seem to really hinge on Rengar's ultimate compared to Kha'Zix's, as well as Rengar simply having fingers, both of which are rather superficial and disregarding who they are as characters. Fights aren't always about appearances, weapons, or talents. It's as much the mindset and goal.
I'm a Kha'Zix fan, and though I admit I'm fairly biased against the one-eyed cat, I'll try to look at this feud as objectively as possible. This scenario can play out countless ways, so I won't claim anything is absolute truth, but here's my input.
In-game: Let's say there are two players, 1v1, masters at Kha'Zix and Rengar that play the Champions to their highest potential. They're both max level, Kha with three of his evolutions, Rengar at max Ferocity with a fully-stacked Necklace, and full build. It could go either way, but I'll submit and say Rengar has a slight advantage. Both exceed at eliminating single targets, both are highly mobile, and both have extreme burst. However, Rengar has better sustained damage and, mainly due to his Battle Roar (W), doesn't require as much precision to deal damage, become more durable, & even heal. I'm not saying it's a guaranteed victory, but Reng has a little more in terms of a duel.
In-lore: Again, there are countless ways this could go down depending on a variety of factors, such as environment, how the two meet, etc. In-lore though, I would say Kha'Zix has the advantage. Why?
- When the two first met, Kha'Zix was at his weakest, having recently just come to Runterra as a larva/nymph/just-underdeveloped, while Rengar was already an expert hunter. Yet still, Reng couldn't kill Kha, and what's more is that Reng was permanently scarred while Kha has not a single scratch from that battle. We can assume Kha has been devouring and growing, so if Reng couldn't beat Kha then, when they meet again the Voidreaver will have become even powerful. Kha is a living swiss army knife that can only get stronger with time, to which Rengar has been wasting by leaving him alone. What's more is that Kha's not just stronger, he's prepared. He's most likely gained traits specifically designed to counter his nemesis. Granted, so probably has Reng, but...
- Kha'Zix is from the Void, a plane shrouded in mystery. Rengar is from Runeterra, a world that the Void has been able to quickly adapt to. When Kha came through, he was weak, but grew stronger and became an apex predator of a foreign existence. And if the Void truly does operate as a unified invasion force, Kha's mission/purpose is adapting to this world, learning its strengths and weaknesses, understanding and killing the opposition. Basically, he's specifically designed to slay Runterrans. Reng on the other hand (as it currently stands at least) doesn't have the faintest idea about the Void or what its denizens are capable of. All he knows is that Kha is otherworldly and he's a big bug, which is a gross underestimation of what the Voidreaver, or any Voidborn, is. Understanding the enemy is more often than not just as important as physically brawling with them, and Kha has much more capability of understanding Runeterrans than Reng can of Voidborn.
- Also pertaining to origins is experience. Rengar has hunted big animals (and got his ass kicked by a Yordle with a bad temper if I may add). Kha'Zix has survived among the likes of Rek'Sai, monsters even the demigod-like Ascended of Shurima dread. Based on that, which is the better survivor? Kha'Zix has the strength of two worlds, Rengar is limited to one.
- Mentality. As said, Kha is perfectly designed to slay Runterrans, not just physically but mentally. He doesn't know what fatigue, doubt, or fear is. He is not weighed down by petty thoughts other than to become more biologically perfect. I don't even think he values his own life, at least to a capacity that we do. He looks to kill and evolve until something outdoes him, without the fear of death. The embodiment of Darwinian evolution. Rengar is prideful (hell, his title is the Pridestalker), concerned with what will come after the battle instead of just the battle itself. Rengar has more to lose, thus more to worry about, thus more to slow him down.
- You state Rengar has the advantage because of hands. I dare anyone to go into a fight with nothing but their bare fists against someone of equal or greater fighting talent wielding two blades the size of your body that are wielded as seamlessly as any appendage and able to cut through anything from flesh to armor, while being imbued with some extradimensional energy that can only be assumed to be the equivalent of a lightsaber. Even if one does manage to avoid these monstrous weapons, they then have to contend with fangs adept at rending struggling prey, spines that puncture then explode with some sort of corrosive Void energy that also renews the users cellular structure, and velociraptor-like talons for feet. Add in a sleek form that blends into the shadows with or without cloaking (as opposed to blatant white fur), the mobility many insects display, a carapace that provides natural armoring but also doesn't slow down, an unknown biology (how do Voidborn breathe exactly?), plus the fact that even if the flesh is pierced (as seen from the Xer'Sai and Diana's fight) is acidic to the touch; I doubt Rengar will be wanting to drop his weapons any time during the battle.
Again, is it a guaranteed victory? Definitely not. But the combination of Kha'Zix being a perfect-born killer, an enigmatic creature, and a nightmare child of a hostile plane of existence that has shattered the sanity of mortals with its mere gaze; Rengar has a lot to prepare for and contend with.
