Well, I took SJCP and I began SJCD. I guess if you ever programmed in Java, many of the topics on the first test are easy. You just need to study some less used subjects. Also, the questions are pretty straight forward: right or wrong.
SJCD in other hand is more complicated. If you already work with the technologies required by the exam (locks, threads, rmi or sockets, swing) it is easier. Otherwise, you will need to "master" them. Also, you need to document everything and most problems require analisys and judgment (and documentation of your decisions), so everthing is a doubt.
On a comparison scale, it is very relative. I would say SCJD is 10 times harder that SCJP. Depending how much you know about threading, locking, RMI, File IO, Exception handling, Swings, it could be lessser. But note, you are at your own pace, and dont have to appear for a live exam, atleast for the assignment. Not stupulated time to finish.