I have heard that the HVAC depth on the PE is the easiest IF you have worked in the HVAC industry. If not then take either machine design or thermo. I personally took machine design, not because I design machines every day at work or anything, but because I felt it was closer to what I did than HVAC or thermo. I think most choose to take machine design by default if they have no significant experience in either HVAC or thermo.
As far as difficulty (at least in my case)....
1. FE exam, passed on 1st try, took during college, total study time of 2 hours (basically took it cold).
2. PE exam, passed on 1st try (MD depth), took exam about 6 years after college, total study time of about 350 hours, and gave myself about a 50/50 chance of passing after the exam.
I personally thought the PE was much harder than the FE.
Good luck and make sure to check this board for tips on a frequent basis. Makes a world of difference.