I think you are trying to compare Apples with Oranges.
The answer to this question depends entirely on your preparation. I found the RBI Grade B Exam tougher when compared to Civil Service Exam, simply because I never prepared for it. The only time I wrote RBI Exam, I did not clear it. That same year I went upto the interview stage in the CSE Exam.
As for RBI Grade B Exam, I applied for it without even checking the pattern for the exam. A friend of mine told me that it was similar to the CSE Exam and hence should be good experience writing it before the actual exam. I was so rusty from not having written any exam for years, I wanted some experience at writing an objective test in an exam environment. I did not want a job in RBI and hence I never gave the exam the respect it deserved. I did not qualify. But it did serve the purpose for which I wrote the exam.
I think I might have done a little better if I had checked the pattern of the exam (the type of questions they ask) before actually attending it. Even then, I don’t think I would have qualified. My preparation was entirely focussed towards clearing CSE. However, if I had put my mind to cracking the RBI Exam, I would have had a fairly good headstart because of my CSE preparation.
Any person who thinks light of an exam because he is good at some other is setting himself up for failure. You can use a sword to cut vegetables, but it is not the optimal way of doing it.