I have taken both courses. I did the Kaplan course/private tutoring before my first attempt at the GMAT. I got a 530.
I then took the Manhattan course + online tutoring with Manhattan, and got a 730 on the GMAT.
In my opinion, there really isn't any comparison between the two. Unfortunately, Kaplan and Princeton Review don't have the same standards for tutors as Manhattan. Manhattan doesn't hire anyone who hasn't scored in the 99% or above on the GMAT (760 or above). Kaplan and PR frequently hire people who have 710 or above, and may have taken the test 6 years ago, or perhaps even longer than that.
That isn't to say you won't learn anything with Kaplan or Princeton Review. However, I found that their strategies rely heavily on process of elimination and plugging in numbers. While these strategies will often get you to the right answer given enough time, the problem is that the GMAT is a timed exam with some very difficult questions. Therefore, unless you are extremely gifted in quant already (and thus probably going to score high regardless on the GMAT), these plug and chug strategies don't work particularly well.
I found that Manhattan offered a much more analytical approach to the GMAt, where you learn the fundamentals and then can apply them towards solving the problems. While this approach is certainly harder to learn than the PR or Kaplan strategies, it will pay off in spades when you go to take the test. The GMAT is, at the end of the day, a test of logic wrapped in math problems (or verbal problems, re: critical reasoning). If you can understand how to break the problems down into logic problems, these problems become much easier to solve.
Anyway, that's my 2 cents, from someone who has done both programs....
