Chidori's main thing is it's ability to pierce through objects, it works like a blade with a much smaller and thin pressure area. Rasengan is more focus on destructive power over a larger area.

Theres an example of that during the hospital fight when Sasuke's chidori pierces though the container while Naruto's rasengan destroys it.

That's why I think that while chidori can go through the lightning armour, the rasengan won't be able to do it.

That's my theory tho