My parents and I have done both, and we enjoyed Christmastime more, but there were many factors besides time of year that made it better. Honestly, the weather was almost identical on both trips. In fact, I think it was slightly hotter in December than in late March/first of April. But that's just luck of the draw. Our Spring Break/Easter trip was our first visit to WDW in 17 years, so we were kind of rusty at how to tour (despite my diligent homework here on the disboards). We were also staying at a moderate resort and we drove. The Christmastime trip we flew, stayed at a deluxe resort, and had solid plans that focused on holiday events instead of just general touring of the parks.
There is nothing like Disney at Christmastime. The Magic is on steroids during that season. But the weather can be iffy as you never know if you're going to get summertime temps with swimming and t-shirts/shorts or a cold snap with sweatshirts and jeans. And it can change from day to day. Spring is more consistent weather wise, but you're more likely to encounter some thunderstorms.
I'd say crowds are about the same. I honestly can't tell much difference between the crowds on our spring trip, our December trip, and the summer trip I just got home from. It's crowded. Period. If you plan accordingly and zig when everyone else zags, utilize FP+ like a boss and hit rope drop for headliners that you cannot get a FP+ for (due to tiers or just bad luck on the day you book), you should be fine at either time.
I'd give the edge to Christmas just because you have the holiday events and decorations. But spring is lovely too with the Flower and Garden. Tough call!