they both have pros and cons IMO:
-Chidori, you rush in blindly, without the Sharingan, you suffer from a tunnel vision effect and are prone to being countered by the enemy if you miss.
-Rasengan, the technique can hurt the user if it goes off without hitting the enemy, in naruto's case it requires the use of clones to help stabilize the technique.
OT: I think the Chidori is a more DEADLY technique, Chidori punches holes through your chest causing fatal amounts of damage any time it hits, Rasengan just unleashes large amounts of concentrated chakra generally subduing an enemy