I've been to both at Christmas, personally I think
Disneyland was the better of the two but both were wonderful. It was the fireworks followed by snow on Main St at DLR that won me over. I also think the candlelight processional is a bit more special as they have the carolers walk down Main St singing before taking their place on stage, I imagine they have different people doing the story of Christmas each year but it was Dick Vandyke when I was there - made my day!
The decorations at both are fantastic, love all the different trees at all the parks and resorts at WDW (can't miss the gingerbread houses either!). Everyone and everything is so festive and fun, couldn't recommend a better time of year to go to either!