October is no longer an off-peak month in terms of crowds. The Halloween Time festivities have increased crowds and made it a peak time to go. It used to be considered off-peak season a few years ago, but since 2008 the crowds have been steadily increasing. I think last year was the busiest Halloween Time season thus far.
Plus, any new attractions or shows will drive the crowds up substantially. World of Color debuted in June 2010. By all accounts, the crowds for the rest of the year were quite substantial. Some folks even said they would not go during October again because it was too crowded and annoying to try to navigate around. This year we have both the opening of the new Little Mermaid ride and the new Star Tours. Next year will be the opening of Cars Land.
Honestly, I don't think this coming October will be any 'less' crowded than May of this year.
The only thing that may work would be to try to go during a less crowded time of October - if such a thing exists now. Traditionally, the first couple of weeks of October are busier than the middle portion of the month, due to various school breaks, a couple of conventions or special events and Columbus Day holiday weekend.
When mid-October (after Columbus Day) rolls around, things quiet down a little bit, but then they pick up again in the few days leading up to Halloween.
Weather-wise, October is very unpredictable in SoCal. It can either be extremely hot and dry (like as high as upper 90's or low 100's) or it can be cool, breezy and overcast. It may also be rainy. It may be a nice pleasant, sunny 70 degrees. The worst combination is dry weather + heat + wind. That results in wildfires, which can then pollute the air. I have experienced all of these weather conditions in Octobers past - I never know what I'm going to get when October rolls around!!
May will probably be more likely to be sunny and warm because it's less unpredictable than October is.
Overall, I would pick October as the time to go just because Halloween Time is festive, fun to experience and should be enjoyed at least once. But I would not count on light crowds during that time - especially if it's any time before Columbus Day.