I did both exams, because I felt they dealt with different areas of server-based software development in the Java world. The SCWCD exam was very easy and covered a good (although somewhat dated) foundation of client-server programming. The content of the SCBCD exam was more difficult to grasp because - at least in my eyes - it included some specific concepts that were less common and required more learning to fully understand it, although I already had some practice with Java EE. If you have never done any real work within the EJB world before, the SCBCD exam can appear very difficult. Still, looking back I think I had more fun with SCBCD and had personally gained more from it.
Nonetheless, if you are doing the exam mainly to improve your job chances, I would vote for the easier SCWCD exam which will show you know the basics. Plus, there are some excellent books out there to prepare you for this exam (which is unfortunately not the case for the other exam, at least when I searched for them). My feeling is there are more companies out there interested in people that know the general stuff instead of the intricacies of the Persistence API.