We're addicted to September and Jan/Feb cruises as these are birthday months for our family. The Sept. cruises are warmer (think summer, but without the oppressive heat), but you are in the middle of hurricane season. This means that the ship sometimes doesn't go where it is scheduled to go. You will have a great trip, but you might visit "alternative" ports. If you are flexible, this is wonderful. We also are scheduled Sept. 25 for our 11th trip on the Magic.
January/Feb. can be cool. It can also be hot. That sounds dumb, but we've had bad weather 2 of 6 trips.