Look, first of all, stabbing someone with a kunai does not instantly end a battle. Second of all, those two justus are very complicated and hard to master, even control. By themselves, you can do a lot of damage.
Rasengan is a dense ball of rotating chakra. A little chakra by itself is nothing, but if you harvest enough chakra to make a rasengan, you could so some serious damage. As your saw in Naruto Episode 95 (I think), Kabuto took a lot of damage when Naruto drove a Rasengan into him. See, Kabuto's cells can regenerate instantaneously, so it's pretty hard to do any damage to him.
Now, I understand your view on the chidori. All we really see Sasuke and Kakashi do with it is plunge it into other's bodies. But still, the chidori requires a user to be able to have advanced chakra control. Just like one of the variations of the Rasengan, the Rasenshuriken, the user has to perform a change in chakra to create the chidori. There are many variations of this jutsu that are better than the basic chidori, like the chidori blade. But still, the chidori itself is really strong.
In conclusion, I don't see why a Rasengan vs. Chidori would end a battle. Besides, massive amounts of chakra can be very reactive when making contact with other strong sources of chakra.