From what I've heard from people who have seen both versions of the Haunted Mansion, the WDW version is better. I know it is longer than the DL version. The WDW version also has the new Hitchhiking Ghosts which in my opinion are great. The only thing from the DL version that I heard is better than the WDW version is the exterior.
The Library and MC Esher Stairs are bonus scenes that MK has over DL. Not to mention the recent refurbishments within the ride and the extended, interactive queue out front. That and I always thought MK's looked so much creepier than DL's.