With nen, Killua has Ten, Ren, Ko, Ken, and Ryu for stat buffs, his Hatsu gives him electrical attacks, En gives him a sphere of influence that allows him to percieve moves within the radius of his aura. His martial arts mastery, body control, weapon mastery with the yo-yo's, and assassination techniques in general are dangerous and make fighting him at melee range a huge issue for Sasuke. Skill wise, Killua his considered one of the more skilled characters in the verse, in terms of fighting skills, being an assassin, and especially nen. At this point in the story, Sasuke is considered a solid chunin tier ninja. He has been since the beginning of the story and by this point he would probably be a high tier chunin but clearly not on the jonin level yet. As for amps, the way Chakra and Nen amp a user is almost the same, though nen is more detailed and explained. When it comes to Chakra, boosting one's physical attributes beyond a certain level is considered extremely difficult even among the most skilled shinobi. Regular amps are like what Sasuke used against Zabuza clones and Haku after learning chakra control or people using body flicker like what Sasuke did against Gaara. However that just became a part of Sasuke's fighting style so the speed and strength we see him and most other ninja operating with is with their chakra amps already on. The prodigous chakra amps are like Tsunade and Sakura with strength or like the Raikage with speed. Those are noted as it goes far beyond what amps shinobi already use in everyday combat. As for range, in terms of attacking at a distance they're pretty equal. Killua can shoot lightning bolts, Sasuke can shoot fire. Killua's nen influence extends further than Sasuke but neither of them are really ranged fighters to begin with. Up close Killua has the range advantage with his Yo-Yo's, but even without that fighting up close is dangerous.