We've done both. But our spring break is the week of St Patrick's day which is kinda early.
The Christmas atmosphere is pretty amazing - all the resorts and parks have some level of decorations and then there's the opportunity to also enjoy a Christmas Party at MK with the pretty spectacular Holiday Wishes.
Spring Break was great for the weather. It was dang near perfect. It was never too cold or too hot, and we were able to enjoy the resort pool every day we were there. We were also able to enjoy the Flower and Garden Festival at Epcot which was a real treat, very beautiful.
But, for the "con" side, the week before Christmas was good weather but not great. We barely got one swim in at the peak of the day and even then it was only about 70 degrees! And it rained the night we were at DHS and it kinda sucked because it got cold real quick and we were wet so we ended up just leaving. It rained during our spring break at MK and it was very different. The temp didn't really change, and we just hung out cause most other people left. I guess people who go to WDW in December aren't looking to swim, but we equate Florida with summer/sun type activities so even in late December we're trying to swim!! And you're from Canada, December in Florida might feel like July in Canada!! We are from Texas and are official Cold Babies.
I know the weather in central Florida is very moody and goes up and down constantly, but that was our experience.
Either way, you're going to have an amazing trip!!