I am going to assume that you intend to take both of them at some point in the future - if this is not correct, then my points may not be valid.
Like SCJP, SCWCD is a theory based exam - learn the correct theory, then sit the exam and answer the multiple choice questions (drag n drop, single choice ...). If you enjoy that style of exam (or if you are just in the right "mind-set" to continue with that style having just done it for SCJP) then you might want to continue with SCWCD.
Alternatively if you are sick of just studying, and want to get your hands dirty with real coding and working on a real application, then SCJD may be more your style.
If you are trying to get certificates now to pad your resume faster / impress your partner faster / get a pay rise faster ... then you can probably study for (and pass) SCWCD faster than you can complete the SCJD assignment (see the JavaRanch SCJD FAQ for thoughts on how long it will take to complete the assignment).
Side note: Given the time it can take to complete the SCJD assignment, and given that there is no timelimit set by Sun, some candidates start the SCJD assignment first, and if they feel they need a break they go off and do some other certification then return to SCJD later.
If your goal is to get all the certificates complete in as short a total elapsed time as possible, then you should be aware that there is a delay between when you complete the SCJD assignment and when you get the results (Sun say it can be up to 6 weeks from when you submit your assignment / sit the exam to when you get the results - we have seen shorter and longer timeframes). So you might want to do the SCJD first, and then do SCWCD while waiting for the results of SCJD.
Hmmm. Having written all this, I remembered that there was a similar question in the FAQ (but not as detailed). So I will now update the FAQ with my thoughts (anyone else answering this question, please feel free to update the FAQ with your thoughts as well).
After reading Andrew's post, I have a few comments to add. Mainly what is your career path? If you are going to be doing web based development, I probably wouldn't waste any time on SCJD and focus on SCWCD and SCBCD. However, if you're not sure what type of java development you'll be doing, SCJD can give you good hands on development experience with the Java language in general.
I'm a J2EE application developer, but I decided to do the SCJD to have a good project to work on and familiarize myself with the new features of J2SE 5.0 as it looks like the new generation of tools (WebLogic and WebSphere mainly) will be moving towards development environments that utilize these new features in the near future.
I would say consider what technologies you are working with now, what technologies you want to work with in the future, and plan your certification path that way. I know from experience that an employer would rather see 1 certification in the technologies they intend to use, rather than a list of certifications in technologies that they don't care about. Just something to consider.
I can only speak for the UK, but the number of jobs advertised for J2EE far outweigh the number of straight java jobs - in fact, I've hardly seen a java job advertised that doesn't ask for some J2EE experience (EJB or JSP or java with SQL).
So I would go for scwcd and scbcd over scjd.
BTW I took the scjd because I was working on a project that used much of the technology so I could do the exam project and my paid work at the same time.