I just passed the OCPJWCD6. I prepared myself by reading HFSJ, the servlet 3.0 spec, the jsp2.2 spec and took the Enthuware OCPJWCD6 mock tests.
First of all, HFSJ is not enough. It covers the main issues and gives you a good sense of servlets and jsp. You need the specs for new things like annotations, async and some minor changes in the JSP. The Enthuware mock tests covers it all and reflects very well the real exam. I scored an average of 72% on the mock tests and 68% on the real exam.
A few answers:
1) the expected number of answers is given, like "choose two answers".
2) answers like "does not compile"etc are not in the most answers but a few.
3) there are a few questions where you have to think like a translator or compiler but not as much as in SCJP6.
4) the mock test is a pretty good representation of the real exam.
5) I'm not sure it is hard to compare.
Hope this helps you.