chidori is definitely a mix of both form and nature because in a previous chapter kakashi explained how it took sasuke a few weeks to learn the nature before he could start the form. kakashi also says that if chidori were just form, then there would be no point in him having invented it. the mix of form and nature is what makes it destructive, explaining why naruto's incomplete although powered up rasengan lost to sasuke's lvl 2 chidori at the end of part 1