I strongly disagree that prep courses are a waste of money. I could not have passed without taking one. Just like with everything else, everyone's learning style is different. Some people learn well independently; other people want interaction. I took Dr. Tom's Machine Design & Materials class, and I thought it was great. I will say this was before the transition to CBT, and I don't know what the updated course is like. But I also feel like I would have passed by taking any course. You've been though school your whole life, and you know what works for you.
Regarding which exam to take. My recommendation is to take whatever topic you know and enjoy the most. Studying for the PE is a grind. Focusing on stuff that interests you makes a big difference in how easy it is to make it through the process. (Same advice goes for work. Focus on working in a specialty that is interesting and enjoyable. But I digress.)
I've always been a MDM, going all the way back to when I was a kid taking the broken VCR apart and playing with transformers to figure out how the mechanisms worked.
Also you said you passed the FE 2 weeks ago, and that your employer wants you to have "the title". I'm assuming you mean having your PE license. Are you a new grad, or do you have the necessary experience already? Does your state allow you to take the PE exam before gaining the necessary experience? Even when you pass the PE exam, you don't get the title until you gain the experience and get licensed.
