I've sailed on the Magic during both October and December, and I'd vote for October for several reasons. It was warmer, especially the ocean temp at
Castaway Cay. The days were longer. The ship was less crowded.
The cruise in December was great, it just made things that much more hectic getting ready for the holidays, since we were gone a week during December. The ship's decorations were great, but not so much that it would play a role in deciding to cruise in December again.
The price was about the same with the Fall Fantasy rates.
Yes, October is during hurricane season, and we did get re-routed. We stopped at Key West on Sunday, and then went on to St. Thomas. I didn't really see this as a negative, as we did a great snorkling excursion on Key West and had a good time on the island. There was also a hurricane in the Atlantic this year in early December, which caused high seas. I don't know if one season is more likely to have higher seas, especially if there are no storms around. Even though we were re-routed in October, I'd say that overall, the seas were less rough during this trip.
I don't think that the ships are re-routed that often. I think that the Magic has changed itinerary 3 times since it began the 7-night cruises, twice in 2000, and once that I can recall hearing about this past year, in 2001. That trip got to go to San Juan and had a second day at Castaway Cay, which would have been just fine with me.
We sailed in early October, so it was probably a bit early for Halloween decorations, if they do that.
I really don't think that you can go wrong. If you stick with the October trip, don't worry about a hurricane. You'll never get caught in one on the ship. Yes, there is a slight chance that you won't make the scheduled itinerary, but I don't see that as a negative.
Anyway, that's my 2 cents. Have a great trip, regardless of when you decide to go!