I am glad I do not have to make the choice yet as my kid is not there yet. I think it depends on which works better for your family. Weigh the cons of both and choose the lesser evil
For me the cons are:
* Summer: crowd level high until later August, very hot and high humidity. It is important to stay well hydrated and avoid heat stroke (which both DH & I had when we honeymooned there in late July- early August.) Also, lots of sudden rain showers.
On the Pro side, parks open late each day, so you can work your schedule if willing. Arrive at parks for rope drop, leave at lunch time for a siesta and hanging at the pool, and return later in the day as it starts to cool down some. There are also available other outside activities outside the parks if your family enjoy them (golf, water activities, beach time).
* Over Christmas: crowd level highest of the year, weather more unpredictable. (however it does tend to be dryer)
On the pro side, there is nothing like Disney decorated for Christmas, and there are extras that you only get this time of year.
For my family, we would probably choose Christmas as we do not deal well with heat & humidity and have a hard time waking up early enough to get the early start before the heat sets in (we are from the Pacific NW, so body clocks 3 hours behind.)