It depends on whether you like Christmas (December) or Halloween (October) better. Both are an excellent time to go to the parks, with fun things to do and see.
October: Halloween decor isn't very widespread; mostly focused on Main Street, Frontierland, and NOS. The biggest attraction draws are HMH and SMGG (there is also the Halloween Carnival at Big Thunder Ranch). Assuming things stay relatively the same, MHP is a lot of fun, but it is an added expense. There are no Halloween fireworks or parade for the general public (assuming MHP staying in DL, they usually present Halloween Screams fireworks and a cute little cavalcade during MHP). Weekend crowds will be high, but weekdays tend to be fairly nice. Fireworks (if they happen) and F! tend to only be presented on the weekends. No one knows what Paint the Night's schedule will be; WoC tends to be nightly. If you go in late-October, IaSW will be closed for the installation of the Holiday overlay.
December: I think the Holiday season is a more immersive experience, with decor in both parks, DTD, and resorts. The biggest attractions are HMH, IaSWH, and Jingle Cruise (Jungle Cruise with the holiday overlay). There are other holiday themed activities as well (e.g., Viva Navidad, Big Thunder Carnival, etc.). At this point, there are no hard ticket holiday parties. There won't be a Christmas Parade this year (because of Paint the Night), which is a disappointment for many guests. I don't know about the Christmas fireworks (it's possible they also won't be presented due to the 60th). If you go in December, fireworks should be nightly, as well as WoC. F! isn't presented nightly until closer to Christmas (so late-December). I've found that weekday crowds are okay, weekend crowds are high, but overall crowds increase as the month progresses.
I've been tracking known events and school breaks that may impact DLR crowds in the
2015 Calendar thread. Many of the October weekends are impacted by events (e.g., Gay Days, CHOC walk, UEA, etc.) as are a few of the December weekends (e.g., Candlelight Processional, etc.).
