I can't give advice on October, since I've never been then. If you and your family LOVE wearing costumes and IF there's a (separate admission) Halloween party on your October dates, then I would consider going in October. Otherwise, I'd definitely go for December. The park is so magical then.
We went in the second week of December 2009, arriving the Wednesday after the Cast Member parties (which are generally, but not always, the Monday and Tuesday after the Candlelight Processional), through Sunday afternoon. Crowds on Wednesday and Thursday were very light. Friday was basically the same, but it was overcast (and started to sprinkle at maybe 1 PM). I think if it had not rained, Friday night would have been busier. Saturday was also not crowded; it rained basically from open until 9 or 10 PM.
One extra advantage we had is that the park was open until 11 on Wednesday and Thursday, and Fantasmic! ran twice. Historically, that early in December, Fantasmic! wouldn't be running on "weekdays" (I'm not sure that it ran on weekdays the following week!). But someone here theorized that the extra hours were a way of "making up" for the fact that
Disneyland closed early on Monday and Tuesday.
It's hard to know what will happen next year, but I think Sunday - Tuesday would be less crowded--if you went Thursday - Sunday, then you'd really be arriving sometime in the middle of the day Thursday, right? So you'd be there over a light Thursday, then a light Friday morning, a fairly busy Friday night, fairly busy Saturday, and probably a fairly light Sunday morning?
There's a decent chance that Fantasmic! will only run on the weekends (and a smaller chance that World of Color will only be running on the weekends), which would likely increase crowds in DCA and maybe DL on those days as well.
That said, if you WANT to see both World of Color and Fantasmic!, then going over Friday and Saturday COULD be a better bet. Hard to say, though.
Have a great time, whenever you go!