So both are really strong jutsu, but it is commonly known that rasengan is stronger than raikiri.
It's understandable but if you look at some situations in which rasengan didn't kill the target, you will see that raikiri would have done a much better job. While rasengan is way more complicated and their are more applications for it it is not as deadly as a raikiri.
example 1:
example 2:
In both of these situations a raikiri would have been deadly, because it is penetrating the body leaving permanent damage.
It's understandable but if you look at some situations in which rasengan didn't kill the target, you will see that raikiri would have done a much better job. While rasengan is way more complicated and their are more applications for it it is not as deadly as a raikiri.
example 1:
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example 2:
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In both of these situations a raikiri would have been deadly, because it is penetrating the body leaving permanent damage.