When collided, they are equal but as to which one is stronger hmm. Chidori has greater killing potential because, as we have seen, you can pretty much put a hole in there chest, but your attack has to be very accurate and the attack leaves you wide open during and after the attack. Thats why it can only be used with sharingan, to see counter attacks coming. Now the rasengan is like a bunker buster, it enters the body then explodes causing severe internal damage, debilitating or killing the opponent, either way they go flying. Even with gathering chakra to block it, it still does serious damage. However it is a close combat technique and is best utilized by attacking blind spots, so it works better with deversionary tactics i.e. kage buunshin, or speed attacks i.e. flying thunder god. Buuut in the end I'ld have to go with the rasengan since its size and power can be increased to rediculous levels.