Can't comment on small group vs. private tutor. Also, I wrote the MCAT during the last paper and pencil administration (I'm old school like that), but from what I've been told, everything I say still rings true.
I took Kaplan and thought it was great. Princeton Review focuses more on reviewing the material in-class - this may be beneficial if you feel you need to brush up on it, or have no background in it, but since it was mostly covered in my undergrad degree, I felt I needed less time with the material and more with mastering the test format. In this regard, Kaplan has a huge online library of practice tutorials, quizzes, and section tests that I found extremely helpful in preparing me.
I am an instructor for TPR, and I've taught both courses and private tutoring sessions for them. From what I've seen, different formats are better for different students. The 1-on-1 tutoring sessions are great if you learn at a very different pace than your peers (much faster or much slower) because the material can be tailored to you. However, there are generally fewer hours of instruction for private tutoring, which means less overall review and fewer in-class passages.
You would need to decide which would be better for you: more material covered or a more personal pace.
I took The Princeton Review course last year and had a few hours of tutoring as well on the side. I hadn't taken ochem at school, so I needed extra help. I had a great experience with my tutor. It really doesn't matter which company you go to, because at the end of the day, it's whether the person teaching you is qualified or not. Speaking from my experience with PR's courses and tutoring, I am very impressed with their teachers.
By the way PR also has an online student center and offers a whole bunch of practice tests, drills and passages. They also grade your essays too.
Having tutored for Kaplan, the one thing I'll caution is that it's a waste of time unless you're very specific about what you want to get out of it. If your train of thought is "Well I could use all the help I can get, might as well get some 1 on 1 time", that won't be the best use of your money. Basically you're throwing it out to the tutor "Hey, can you just randomly pick and choose from a year's worth of gen/org. chem, bio and physics, plus verbal tips, so I can best succeed on the MCAT?" I had a student like that, and I had no idea where to start, so I just hit high yield topics from each science with her. And I could tell at the end of the session, she only retained 1/10th of it. If you go in knowing you're really weak on say osmoregularity, acid/base titrations and DC circuits, a good tutor could definitely have some value.
Can't speak on TPR course either, but Kaplan is pretty good. The online stuff is pretty solid on it's own, and if your instructor is engaging, the class can be a nice bonus. You really need to be able to dedicate the time for it though. If you just show up to class and run through the required quizzes, you're really not getting anywhere near what you could. But to get the maximum output, it is a serious time commitment.