Strengths:
Hard CC via bola, Rengar doesn't really have that hard of a time ganking pre-6 get up to at least 4 stacks before you gank and E-E which is most likely a kill or a flash if your laners follow up
Quick burst, Rengar currently has the fastest burst out of all assassins he can delete you quickly before you can even react. If played right Rengar can take at least 1 champ out of the fight resulting in a 5v4, but to do this you have to learn his combo and become consistent with it.
Medium-fast jungle clear, Rengar can survive easier in the jungle than Kha'Zix because of his W sustain and resistances
Highly mobile in the jungle, When Rengar gt 6 stacks on his bonetooth necklace he gains 125 range on his passive and if you master his jump you can engage on a target easily from the bush
Ganking potential, Rengar's ganks are okay pre-6 because of his bola you can sit in the top or bottom brush and perform a lane gank with it at 5 stacks. When Rengar reaches 6 ganks are easier and more reliable, you can choose to instantly root the target or if you're fed enough instantly burst them.
Weaknesses:
Ult is useless against coordinated teams, teams that are huddled up is not good for Rengar unless you're fed Rengar preys on ungrouped enemies. He is also countered by 
Squishy, Rengar is pretty squishy because he's built glass cannon(full damage) and if he doesn't successfully delete his target he'll be deleted himself which can be really bad for your team. You don't want to pick Rengar every game, make sure you have at least 2 tanky champions on your team before you pick him
No escape, wether you killed you target or not once you go in you have no way of getting out so you most likely die unless the enemy team is distracted or you flash out
Strengths:
Good ganks pre-6 with E
Picking off multiple targets, unlike Rengar if Kha goes in to assassinate he has a better chance of escaping because his E resets and gain range on enemy champ takedowns so even if you're not the one to kill the target you still get the jump reset. Kha can you this reset to jump on another enemy, take them out and repeat. Kha can also double jump if you practice and learn this skill: https://www.youtube.com/watch?v=hq598fuJbTw (Sry for the bad quality)
Great engage and escape, with Kha's ult and evolved E kha has a reliable jump he can use anywhere anytime unlike rengar
Low cooldowns, Kha has low cooldowns on his abilties especially on his main damage source Q which is about 3 seconds and cooldowns get lower because of these items 
Weaknesses:
Squishy, Kha gets beat up hard early by the jungle and can get easily counter jungled. Make sure you have at least 2 tanky champs on your team before picking him this applies to Kha as well as Rengar
Terrible damage early, Kha got nerfed a long time ago and this hit his early jungle hard your Q barely does anything to monster and champions which is why his jungle clear isn't so great. Kha needs to snowball a little to be useful if Kha is doing terrible he's pretty much useless in the game
Low mana, although Kha has low mana on his main damage ability spamming his abilities will man you run out of mana quickly due to his low mana pool
Summary:
I like both champs and I think it depends on who you find more fun. I used to play Kha a lot more than Rengar but Rengar is more appealing to me right now. Kha just isn't in his prime anymore like his used to be, but when played right he can be an unstoppable force. I still play Kha from time to time but it's more fun learning Rengar's burst combo as I'm still not consistent with it. I hope I helped! :D
