November seems to be better because the earlier you apply the better, and the sooner your scores reach the univ., the earlier they can decide. So don't delay taking the test. In terms of preparing, atleast for me, one month wouldn't make a difference. I realized that i end up just postponing until i have just enough time. so once u set a date, start focusing. Once u register for the November one at least you'll have a mind set that the gre is pretty soon, and hopefully u'll start preparing at full pace.
Bottom link: Don't delay until December if you don't have a really good reason to do so.