I can't tell you specifically what the differences are for the 11th edition but I do know that the 11th edition was updated for the breadth/depth type format when the MEPE exam switched a few years ago. The MERM still has the same information (I don't think they removed anything) but they added stuff for the new format. There are still many problems that are much more detailed and longer than you will see on the real exam. I actually skipped all of the "Time limit: 1 hour" practice problems when I was studying, it didn't hurt me during the exam.
You will need the practice problems book because it has the solutions to the practice problems found in the MERM. Only the answer to the problem is given in MERM, no solutions are given except for the occasional example problem.
You will want to order the NCEES sample exam and work that just like a real exam, about 3 weeks before the actual exam. Lock yourself in a room for two 4 hour sessions and work the problems with your references. Take notes while you are practicing of things you want to make sure you have during the exam and references you need but might not have. Good luck!