The answer ultimately depends on how difficult you found the CWI test to be, and I have found that many of the same people who have a CWI are successful with API examinations. The 570 exam structure is based upon several more documents than the CWI, and you need to have a greater knowledge of piping systems, metallury and inspction practices specific to piping.
All things being equal, if I had to retake my CWB, ASNT, CWI, API 510, 570, or 653 certs, my least desireable choice would be the CWI practical because the questions were so poorly worded, and the replicas and tools were, frankly, horrible.
To answer your question, the 570 could pose a greater challenge to most as compared to the CWI, but having said that, I do appreciate the motive for your question, and suggest that you go for the 570. Classes offered by Code West, among others, will help you obtain this valueable certification.
I had good luck studying 570 on my own, then testing -- got a PASS with a respectable score. Probably would not have passed the CWI exam using only self-study; had a 10-day 20-hr seminar provided by Bechtel, made a 'comfortable' score. Thus, IMHO, CWI > 570.
Further, 510 is a lot tougher than 570; 653 somewhat tougher than 510. Take a seminar if passing on the first try is important to you.
