The core difference is that the scjd is pratical programming, the architect exam is a design/concept exam. I have worked for most of my time in configuration management so the developers exam was a real challenge to do so hands on programming again. The Architect exam on the other hand allows me to practice what I preach to the developers under me. Use patterns, use the best tools for the job, use the most appropriate technology e.t.c. With of course a sun twist to the way the questions are asked, but give me Solaris, and cross platform open source and I'll be happy. Sounds great coming from someone who got his mcsd before even starting to learn Java.
The programmer's exam focuses on the fundamentals of the Java language. The developer's exam goes further and includes a pratical design/programming exam. The architect test focuses on architecture and high-level design. It depends on what kind of work you do and what kind of skills you want to gain and what kind of skills you want to certify. I've done programmer and developer and I'm working on architect. Personally I don't see how you can do high-level design and lead a team to implement your design without understanding the low-level coding issues as well. I feel strongly that the boss needs to be able to do the jobs of those underneath him to be able to manage effectively.