SCJD is a development assignment that encompasses design,
patterns, Swing, RMI, and language fundamentals. It is also large enough that it takes a significant comittment in time.
I havent heard of SoftSCJD, but after finishing
SCJP, I registered, downloaded the assignment and procured a copy of habibbi's SCJD book, which has everything I have needed. This book is centered around a fictional assignment and each chapter is annotation that desribes the design decisions and key technologies. It is close enough to the actual assignment that I dont think anything else could help you more. Of course get a design patterns book to supplement.