If you get a good DIS group going and do your own Halloween thing it is incredible. Wasn't that impressed with
DCL's trick-or-treat; we were thankful we did our own, the kids loved it.
The adult party/costume contest is fun and the ship is nicely decorated for Halloween (atrium, balloons, entrances to restaurants, etc.)
I don't like Mexico as much as Grand Cayman or St. Maarten/St. Thomas so that would play into my decision. Although it is technically hurricane season, there are few late Oct/Nov storms - I would think the Eastern might stand more of chance as the water is warmed because it's farther south. However, you could pick either and end up with the other IF there is a hurrican as they try to avoid the area.
Is your family a beach family? If so, the two trips to Castaway would probably please you more. Do you like excursions? What kind - ruins would lead you towards the Western, boating adventures would lead me to the Eastern.
Both are great IMO! We didn't have to choose this time because we wanted New Years so we're doing Western again.