Ah, I say they are both very very VERY fun right now. I'll tell you how it feels to play both since I love them both. Kha sadly is a bit underpowered right now mostly because of his collossus isolation range.
Kha'Zix is more of a hunter for sure, because when dueling you basically toy with them with your Q, AA, R, W, R, AA, Q combo. Going invisible and appearing behind them or pro el jukes.
Rengar is alot of fun in the fact that he literally comes from nowhere and deletes people in lane. His combos are alot simpler as you just mash Q most of the time, but he has alot more hard-to-learn mechanics when it comes to his kit itself.
Both are mechanically demanding and very fun to learn, both can survive the jungle if played correctly. Look up some builds and guides and see which ones catch your fancy!