Hi Mark, it's very difficult to answer your question, because this is not an "apple to apple" comparison, so to speak.
I have no experience with the CPA exam. I have obtained my CIA certification, but that is quite different, since that consists of 3 different levels that must be passed, and people usually take 3 years on average to complete that.
The CPA is different in that you must pass all 4 sections in a 30 month window. The positive appears to be that once you pass one section, you need not retake that section again.
In the PMP, however, even if you pass 1 or 2 areas but fail a third, you must retake the whole exam, as it is not portioned out into sections.
Of course, there are also prerequisites for PMP and CPA which would be specific to the certification.
But beyond the general "exam format/ rules" related points, the "toughness" of an exam varies widely per individual. Someone could find the PMP incredibly easy but the CPA very tough because they are just naturally quicker with the concepts covered in PMP vs. CPA (or vice versa).