Gaara hadn't been defeated, his transformation had just been stopped. He could have let Shukaku free and killed Sawsgay if he wanted to, but didn't feel the need to until Naruto whooped his ass. Rasengan is more powerful because it is more effective. The Chidori is only dngerous if it hits you in a vital area, otherwise it's no more dangerous than a kunai. It basically is nothing more than an electrified kunai. The Rasengan will wipe you out no matter where it hits you. And you will be finished completely as far as that fight is concerned. The Chidori is only superior in cutting power. It is inferior is AOE and practicality.