Rasengan in it's most basic construcion is a superior jutsu to Chidori. Rasengan (standard) is pure shape manipulation while Chidori is shape and nature manipulation. Rasengan is superior because of the fact it's shape manipulation at it's highest level. Controling rapidly rotating chakra within a confined shpere releases an explosion (not in a BOOM sense but in a small contained form with emense power releasing upon impact), and the fact is Rasengan doesn't require large amounts of chakra (until nature is added).