Hard question. For Chidori, you have to learn lightning affinity, learn to focus chakra in your hand and change your chakra into lightning; you use both shape manipulation and nature transformation. The Rasengan is hard to learn as well since its shape manipulation is stated to be extremely advance chakra control.
2 things should be mentioned though, Sasuke had the Sharingan to help him learn to use Chidori faster than it would take others; he used it to copy Lee's taijutsu so why wouldn't he use it to copy Chidori which also needs the Sharingan to be effective? Also the Chidori is a completed jutsu, FRS is a completed Rasengan since FRS is Naruto adding nature transformation to the Rasengan while Chidori has both already. In that case I go with FRS being harder.