Hey there Megs,
First, you may want to read this post:
http://forums.studentdoctor.net/showthread.php?t=183386&highlight=kaplan
But to really answer your question, it may depend a little on your background. If you're an english major, for example, and never took any of the sciences on the OAT, then you may want to take the earlier class to give yourself more time to learn and review stuff.
I took the class offered in August-September and took the October OAT. I was a psychobiology major however, and that two or so weeks left before the actual exam was probably more than enough time for me to re-review everything after the Kaplan class ended. I should also mention, however, that I was only taking one class at a J.C. at that time, not working, and spending LITERALLY all my free hours at the Kaplan study center for those two weeks just watching videos, taking practice exams, etc. (see the post I mentioned for more on that). All righty, hope that helps. Good luck!