I bought the Kaplan Premier
last June. I can't say the difference between the two products. However, one good thing about the Premier (I'm not sure of it's present in the other product) is the monthly online quizzes. I took the GMAT last October and bombed.
When I was reviewing to retake the GMAT, I realized that some of the questions that the monthly online quizzes already contained some of the questions that were asked during the actual GMAT. I think that reviewing the last two monthly quizzes can give a reviewer a leg up when the actual exam comes because you already see some of the questions that will be asked.
For example this question from the Purple Quiz came out on the GMAT I took:
While flying over the Pacific, an airplane makes a 25 degree turn to the right to avoid a storm. If, as a result, the airplane in traveling in a direction 5 degrees east of north, in what direction was it originally flying?
A. 30 deg west of north
B. 30 deg east of north
C. 20 deg west of north
D. 20 deg east of north
E. 5 deg west of north
This question took me totally out of my rhythm and cost me a lot of time. The online quizzes from Kaplan
can give you a sense of the kind of questions that will go your way on the actual GMAT.