Boy that's a tough one. I think that the SCWCD is a bit tougher than the SCJP 1.4, but now that we have the SCJP 5 exam I'd say they're pretty close, and I think that they're both a little tougher than the SCMAD and the SCBCD.
As far as I know Sun doesn't any pass/fail statistics to the public, but I know they like to see a fairly high failure rate because they want the certifications to really have some value.
Spot false dilemmas now, ask me how!
(If you're not on the edge, you're taking up too much room.)