This one I would not even have to think about. Usually we go both in September and in January. January wins hands down. September is still hot and in fact we have had it be much hotter than our June trips. While we have just started going in September we have always gone in January. We have never had a full day of rain in January. We have also only had two trips that were even what we would call on the cold side. Most Januarys are still warm enough for us to be in shorts and sandals. (we are from the north so we do not mind it being cooler). We do sometimes need light jackets in the mornings and evenings.