I have only taken Kaplan, so I can't offer any insight about Princeton in-class sessions. What I can tell you is that no matter what prep class you take, the review material will be about the same. All will encompass the basic science stuff you need to know for the exam, plus give you the chance to take practice exams. In my opinion, it all comes down to the instructor.
I signed up for a Kaplan deal, sort of a buy-one-get-one-free, where I could take all of the April exam prep classes and then prep again for August free. I was planning to take the test in Aug, so I figured twice the reinfocement would be helpful. The only problem was the instructors!! The person teaching the April class was a total ding-bat. He was just reading straight from our prep book, and to top off the monotone voice, he was goofing formulas and facts. I left the class bored, frustrated, and more confused that before I came in.
HOWEVER, the person who taught the second class was AWESOME. She was an MD, and had tons of tips and useful outside of class stuff that really helped the review info stick. She also took the time to calm pre-test nerves and gave us useful exam strategies. I felt cool, calm, and collected when I took my exam. So, my best advice to you would be to ask other pre-meds in your area about who their instructor was and how they liked their teaching style.
Also call the center and ask them who is teaching which sections. At the end of our class, we filled out surveys giving our rating of the quality of instruction. I am not sure if the test prep centers will be willing to give you an instructors stats in they suck, but if the person got rave reviews, I doubt they would hide the info from prospective students.
Hope all of this rambling helps you!