As can be seen here there is a big difference between Rasengan and Shidori. WHile shidori is a straightforward assassin ability, which penetrates the tank right from the start, Rasengan has next to no impact on the tank in the beginning.

http://www.narutobase.net/manga/Naruto/176/6

But as can be seen here:

http://www.narutobase.net/manga/Naruto/176/10

Rasengans impact hits most after some space. Following a chidori is more usefull on an direct impact on someone. While Rasengan reaches it's peack just after a few metres after the impact. Following Chirodi will hit a person more as a rasengan, as it penetrates someone easily.

But now how about the susanoo?
While using Susanoo the caster is hidden behind a shield of chakra which attacks, and defends him. For example with a perfect Susanoo or a ripcage.
http://naruto.wikia.com/wiki/Susanoo

But as Rasengan hits just after some space, it shall be a perfect counter, which develops its full potentional after the Susanoo shielding someone. If this truly works, it explains how Naruto will defeat Madara. Of cource Madara said he won't use susanoo anymore, but I wouldn't trust him tho.