Howdy : )
Those numbers are definitely kept confidential. I can give you vague impressions, though and you can draw your own conclusions.
1) Newer exams tend to be *easier* than exams that have been evolving for a long time. SCWCD is new, SCJP is quite old (even though it's on a new version, it's still a relatively *old* [internet time, baby] exam).
2) Look at the style of questions in the SCWCD, and compare them to the style of questions in SCJP. For example, the SCWCD has very little in the way of tricky, complex code examples. SCJP has a LOT of that.
3) SCWCD is a very targeted, tighty-focused exam, with very specific objectives. The SCJP objectives cover a very wide range of potential topics, many of them completely unrelated to the others, except that they are all part of the language.
4) Lots and lots of people sign up for the SCJP more than once. Is it because they just really love it so much they want to do it again?
p.s. I guess it's not too soon to announce that Bert and I and someone else who co-wrote the SCWCD are writing a certification study guide for SCWCD, for O'Reilly.
Won't be out until next fall, though, as we're waiting for the new specs to be finalized (which will probably happen at JavaOne in the summer).
cheers!
Kathy