I would still go with December, because the atmosphere and weather is much better!  The crowds are not that much different, and are still manageable.  Whether you get your money's worth is up to you, not how many people are in the park that day.
Disney has a tendency to run attractions at a certain level depending on the attendance levels. So, for instance in Sept. when we were there they were running two cars for Rock 'n Roller Coaster and there was a 10 minute wait.  Essentially a walk-on, because you walked right into the studio.  During our trip in Oct., the crowds were heavier, and there were more people in line for RnR Coaster, but they were running 4 cars, so the wait was about the same.  The hours are generally longer and the attractions are usually not down for rehab in December versus September.
As always, your mileage may vary!