I've been to WDW at Christmas and will be visiting DL for Christmas at the same time.
I did the Christmas Party last year and the only things really exclusive to the Party was Mickey's Night Before Christmas stage show, Holiday Wishes fireworks, the Christmas parade, and all the cookies and hot cocoa you can eat, a free picture with Santa..
It was fun but costs about $50 per person. After the 24th I believe, you can see all this at WDW for free not including the cookies and hot cocoa and the free pictures with Santa. But after the 24th, the parks are packed to the brim with people and their wheeled torture devices.. I mean strollers.
So I think DL is more value for the Christmas atmosphere.. including the holiday versions of Haunted Mansion and It's A Small World. The only holiday layover we have is Country Bear Christmas which apparently won't be installed this year.