Shouldn't Naruto use Kage Bunshin to hold an enemy in one position, then create the Rasengan inside the guy's head? That would be instant kill, lol.

Seriously though, Jiraiya wasn't even trying when he used Rasengan. Also, it doesn't use as much chakra as Chidori, so it's a more effective weapon. Damage wise, Chidori is an explosion which requires high-speed movement and somewhat to a degree Sharingan. With Rasengan, it's rated "A-Okay by idiots" because of no hand seals, lol, and if used to it's full potential, it can be quite dangerous. We haven't seen the full power of Rasengan, so we can't really say.