I don't want to sound BS, but to me, Kain beats him easily or he outright stomps.
Kayne specializes in killing foes of a larger tier, indeed, but even if we ignored Kain's DC and durability (Considering Kayne's specialty in killing stronger foes), we need to take a look at his speed: Supersonic with Massively Hypersonic+ reactions, while Kayne is simply eye blurring. Kain is faster than him by a huge differnce.
Kayne's arsenal is superior to Kain, but only thanks to his haxed sword. His armour is below Kain's durability/attack potency, so, yeh. But Kayne's sword... Oh man, if the sword is as strong as its durability, then Kain would be cut open like butter by that sword. But even if it had such strength, I can't see Kayne moving a single centimeter of the blade before Kain blitzes him.
Sure, Kayne's blade can block most of Kain's attacks... Most.
The durability of the blade is surely enough to resist all of Kain's attacks, but there are some of Kain's powers (Chaos Dot/Theory/Method/Dynamic, Nether & Chaos Manipulation...) that simply can't be blocked (At least, not by conventional means). Also, Kain has some ridiculously high hax. Chaos Theory creates a rift on time-space that instantly destroys the enemy. Dynamic Chaos shields Kain and auto-fires against any enemy nearby. Nether/Soul Manipulation can pretty much mess up with Kayne's soul and body. Chaos, Probability, Casualty and Logic Manipulation... Even though Kain can only manipulate Probability, Casualty and Logic on minor degrees, these 4 are simply pure and raw hax.
So, yeah. I don't want to sound BS, and my opinion may not count (Considering that I am the creator of Kain...), but Kain takes this too easily.