Why Use Nocturne Over Rengar?
TL;DR: While Nocturne certainly has some advantages, both Rengar and Nocturne fill the same general niche, and Rengar is a superior choice because his advantages are more beneficial to the role that they both fill, leaving Nocturne outclassed.
RENGAR ADVANTAGES
- Rengar has better pick potential (Ult reveals, his burst is higher, his sustained damage is higher)
- He's better as an assassin for the above reason
- He's a better bruiser due to superior innate tankiness and mobility steroids
- His CDs are lower
- Rengar scales to end game fairly well; Nocturne falls off considerably
- Rengar scales better at level 6
- Rengar pre-6 has better ganks than Nocturne pre-6
- When fed, Rengar can carry harder than a fed Nocturne
- Due to his reveal, his objective control is arguably superior
- Rengar's pick potential is less vision-reliant due to its reveal aspect.
- Rengar's ultimate doubles as an escape, and even if used to initiate, grants a MS boost, making him arguably harder to stop mid/end game.
- Because of the aforementioned stealth ultimate, Rengar's split pushing is still superior to Nocturne's.
- Rengar's E can be harder to avoid, whereas champions can use their escape or Flash to break Nocturne's fear tether.
NOCTURNE ADVANTAGES
- Nocturne has superior jungle-clearing sustain
- Nocturne can be a superior duelist against ability-reliant bursters (Due to W Spellshield) when played properly
- Nocturne can clear the jungle faster early game
- Nocturne's ultimate denies vision to enemies, which is highly underappreciated
- When Nocturne catches a squishy out, there is usually very little they can do to avoid death (Other than preventative measures; IE not being caught at all)
- MS boost isn't tied to his ultimate
- His spellshield makes avoiding his pick attempt more difficult if the target in question lack Flash or a built-in escape mechanic, such as Jinx, Ashe (Whose panic ultimate can be negated by W), Varus, Kog'Maw and so on. Effectively, this allows him to negate peel, something Rengar can only attempt to do by snaring and hoping they lack hard CC. Rengar's MS boost alleviates this to an extent.
Overall, I think it's pretty safe to conclude that Rengar is the superior pick assassin/dive bruiser. Nocturne could use some help being given a reason to be chosen over Rengar, because currently, there are too few reasons to choose him over the jungle cat.