While some of the elements between the two parties are fairly similar, to me the parties felt very different. Both were a lot of fun and I can't wait to go again (to both).
Parade: As many have said, WDW's Boo-To-You parade is much more elaborate than DL's Cavalcade. Prior to the WDW parade, the Headless Horseman rides the parade route. It's really cool! I love the Haunted Mansion sequence in Boo-To-You (the dancers with the shovels are amazing). The DL cavalcade is cute, with lots of characters in costume.
WDW
DLR
Fireworks: In this case, both shows are really good. DL's Halloween Screams has Zero flying (in place of Tinkerbell or Dumbo), but he's really neat. Jack Skellington is the host, so if you're a Nightmare fan, that should be a treat. I don't think anyone flies during WDW's HalloWishes, but they make really cool use of the castle and "surround fireworks". Some of the music is the same between the two shows.
WDW
DLR
Trick-or-Treating: WDW has treat stations with one big treat trail. DL has almost entirely treat trails (multiple treat stations along the path). In both cases, the trick-or-treating is pretty popular first thing at the party and tends to be more manageable by the second half. The treats themselves seemed pretty similar.
DLR (WDW was pretty similar):
Dance Parties: WDW and DLR both have character dance parties, which can be fun if you like to do that. I think they're nearly in the same places: Tomorrowland and Frontierland.
Castle Forecourt Show: WDW has a Villains Mix n Mingle show on the castle forecourt stage. Afterwards the villains come down and do photo ops with guests. It's a great chance to get photos with Maleficent, Jafar, and other villains. DLR doesn't have anything quite like this, although I did see some villains at the character meet and greets near Small World.
Character Meet and Greets: Both parties have a lot of character meet and greets. You'll find some "rare" characters out (e.g., the seven dwarfs). I have to say that Jack Sparrow has a much cooler location at DLR; in front of the Columbia with the "fog" coming off of the Rivers of America.
DLR:
Rides: At both parks, most of the big rides are open during the party. DLR has Haunted Mansion Holiday and Space Mountain Ghost Galaxy. None of the rides at WDW have a Halloween overlay.
Food: In both parks not all of the food venues remain open during the party. If you want to eat, I'd grab dinner before the party starts! Be sure to check out any special Halloween-themed items. Last year, they had Mickey and Donald pumpkin cups (both parks) and a Nightmare popcorn/sandwich container.
DLR:
This is a link to Allears website for DLR's Mickey's Halloween Party:
http://allears.net/dlr/tp/dl/mhp.htm
And the link to Allears website for WDW's
MNSSHP:
http://allears.net/tp/mk/mnssh.htm