The requirements for instructors are different depending on how desperate the particular Kaplan center is for teachers. In a city with many pre-meds/med students there will be tons of people with 90+ %ile scores, so anyone interested in teaching a particular section will need to score at or above the 90th %ile in that section. Not so in areas with a deficit of high-scoring MCATers.
In any case, this point is mostly moot since one's score on the MCAT does not in any way indicate how well that person will be able to teach the material to someone else. Hell, scoring well on the MCAT doesn't even require that much knowledge- a lot of it is just good instinct about how to quickly eliminate absurd or impossible answers (much more difficult to teach than stoichiometry or the digestive system).
The most important thing to look for in your teacher is experience. Don't be afraid to come up to your teacher after class and discreetly ask him or her how long they've been teaching for Kaplan. If this is their first class- do yourself a favor and switch to a class with a more seasoned instructor. The first class for a teacher is usually a bit rough. This is because it's difficult to get used to Kaplan's methods (essentially reciting from the Teacher's Edition), getting used to teaching in general, and learning to read the students and amend the lesson to their needs. Many mistakes will also be made during the first course, due to the teacher's imperfect knowledge of the subject matter or errata in the Teacher's Edition.
Also, I don't know much about how Princeton Review does things, but there are several good things about Kaplan: Tons of practice materials, lots of CBT exams, AAMC licensed material, etc. A talented and experienced teacher will also make the classroom sessions fun (the most conservative interpretation of the word) and worthwhile.