Natrajan got his nick name as Yorker king and he is highly effective with Bumrah which adds deadly combo to the pace bowling. Saini os good bowler but inconsistent makes them to be on the lower side and it affect team momentum when they are playing against best teams in their own ground.
Comparing Natrajan with Bumrah so quickly is not right at all. Bumrah has proved himself through his consistency in the long term while Natrajan is just a beginner in comparison.
In the IPL, he took wickets consistently, but also leaked a lot of runs since many of his yorkers were full toss balls. He needs to prove himself as a consistent bowler in the long term.
I have a feeling that Natarajan would make an excellent pace bowling duo with Jasprit Bumrah. Saini is too inconsistent, and he always attempt to bowl as fast as possible, rather than trying to maintain good line and length. He is very expensive, especially when playing against good teams such as England and Australia.
I never liked Saini's bowling from the beginning, but many posters here appreciated his bowling. I feel there are better alternatives out there.