What's the destructive power of the Rasengan about? When you get hit by the Chidori, one of the damages you get is you get electrified since it's made of electricity.
What about the Rasengan? Since the Rasengan isnt made of electricity, what's the damage it's supposed to make? Does it burn? Is it sharp and by rotating you get cut? I discount that the damage comes from the impact, because since it isnt thrown many times and it isnt propelled, the damage and inertia of the Rasengan would be similar to a punch, and it wouldnt make sense to create a technique that does the same damage than a punch.
What's the destructive power of the Rasengan about?
Well, Rasengan being an incomplete jutsu, had no elemental chakra nature until amplified by Naruto Uzumaki.
As you've said, Chidori being of Raiton (Lightning Release) electrecutes the user upon hitting them. If hit with a Fire-Style jutsu, you'd burn. The damage received goes along with the elemental chakra nature the jutsu is of. But as I've already said, Rasengan is an incomplete jutsu and had no chakra nature, simply a spiral ball of chakra.
But because Rasengan is chakra compressed in a ball, spinning and/or rotating at incredibly high speeds, upon being hit with it, a wound that is sprial appears wherever the enemy was hit. This was shown when Naruto hit Kabuto with a Rasengan. On other cases, the enemy is sent flying.