Nocturne has the advantage of a passive that helps his clear, a build path that can either go assassin (warriors lethality/crit) or bruiser (bloodrazor triforce steraks). I've found both work well when ahead but as previously mentioned he can be counter pretty easily.
Rengar has worse clear until tiamat, but far stronger gank in pre 6 and doesn't rely on his ult even after that, plus his passive is terrifying once he has a couple of items.
Really Rengar is better for early ganks, and Nocturne has the farm advantage. After that it's down to the players.