There used to be no real difference between the two weeks. In the past few years, though, October has become SLIGHTLY more popular. Popular enough so that October stopped being included in free dining. Probably not much difference in crowds or weather, but I don't think there were any deals for October last year. If you're really set on the first of October, find out when the September deals end, and check in that day. You will get the deal for your entire stay, even if only your first day falls within the deal period. It will be close to (if not) the end of September, so check in then and your stay should run into that first week of October.