I agree with ikesdsu completely. Depends on what type of work you are doing/ have done recently and/or what subject areas you are mos comfortable with. I was comfortable with both Machine Design and Thermofluids areas, but I picked Machine Design, since I wanted to review some of that material...
A Note on my experience with the exam:
I took ME (Mechanical Systems) exam for the first time in Oct. 12. I was overwhelmed with work during that time and did not get a chance to prepare much at all. I was able to review MERM (for vibrations) and a Thermodynamics text (Moran and Shapiro) only...