MCAD is designed for developers that will never do analysis and design for applications; they just code.
MCSD is for developers who have a firm grounding of development with .NET (you have to pass all the required MCAD exams to get the MCSD), but add an additional exam:
Exam 70–300: Analyzing Requirements and Defining Microsoft .NET Solution Architectures
which tests the knowledge of Microsoft analysis/design methodologies.
With MCSD (unless they change it) requires one extra elective exam as well
(NOTE: although MCAD requires one as well, you can substitute a web/windows development exam instead of the other electives like sql server, biz talk, etc.)
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