I am teacher who travels to WDW at least once per year. I would vote spring break because:
-It isn't as busy, since spring breaks do vary across the country and here in Michigan, we only coincide with the Florida locals if Easter is in the first weekend of April.
-You are guaranteed warm weather to use pools, enjoy water rides, etc. I have been there at Christmas 3 times and have only been able to enjoy the pool on a few warm days. If I am traveling in the winter, I want it to be warm!
Christmas is beautiful and so fun with the decorations, parades, and parties, but be prepared for it to be very busy. Like, at capacity busy if you are still there New Year's Eve. You would need to make fast passes at EXACTLY 60 days ahead of time and you probably still won't get fast passes for newer rides. Depending on where you stay, the bus rides can be extremely long due to traffic and you may have to wait for several busses because of how full they are.
I am going this Christmas, so clearly it doesn't keep me from going

But I go with the mindset that I will only get to ride my fast pass rides and might not spend too much time swimming with my kiddos.