I had to think about this for a bit... IOA placing higher than the Magic Kingdom?
But when I break it down, Islands is actually a very strong contender.
Nostalgia? Magic Kingdom wins... for me. For others, maybe for them nostalgia is Spiderman and Harry Potter.
The Magic Kingdom comes packaged with three other theme parks, two water parks, and tons of resorts. Islands comes with one other park and a handful of resorts. Magic Kingdom is the clear winner in this category.
Coasters: I have to give this one to Islands, as they have the Hulk, the Dueling Dragons, and the Hippogriff, serving a wider range of interests than the Barnstormer, Space Mountain, and BTMRR. I can't count the 7 Dwarves as a real coaster, given that it slows down the moment it begins a descent. It's really a slightly speedier than average dark ride.
Water rides: Magic Kingdom has nothing even remotely resembling the two flume rides at Islands. Splash Mountain and the Jurassic ride are very similar. So, this one also has to go to Islands.
Shows: I'm giving this one to the Magic Kingdom for sheer variety. Plus, Sinbad is pretty awful, and a lot longer than any of Disney's shows, and not even air conditioned.
Parades: Magic Kingdom wins.
Fireworks: Magic Kingdom wins.
Food: Mythos is far better than any restaurant in the Magic Kingdom. However, the Magic Kingdom offers more variety and varied theming. It's a draw.
Theming: Everyone likes to point to Harry Potter and claim it's the only well themed section in the entire park, but I don't believe this is true. Islands is a gorgeous park in its own right. I'm calling this one a draw, though personal taste will dictate your opinion on the matter.
Cleanliness and Service: A draw. The people who work at Islands are the same people who work at Disney, some of them literally. Their training is very similar. One of my favourite moments was walking into Guest Services at Islands and having a young lady jump up and say to the other team members, "Can I help her? Please? She'looks be my first!"
Meet and Greets: This one goes to Disney.
Playgrounds: Personally, I prefer the Jurassic playground to Tom Sawyers island, but that's a matter of personal taste. It's a draw.
Dark rides: Disney has more of the slow moving variety, such as Winnie the Pooh. And the Haunted Mansion is far better than any of the Seuss rides. But Islands has the Forbidden Journey and Spiderman. I'm giving this one to Islands, because the Forbidden Journey is my favorite ride EVAH. Ymmv.
Drop Rides: Islands has Dr. Doom. The Magic Kingdom has nothing in this category.
Shopping: Magic Kingdom offers far more variety.
Games: Sorcerers of the Magic Kingdom offers more entertainment and replay value than the interactive wands, plus, free cards! This one goes to Disney. Though I still love my wand.
Broadness of Appeal: I think Islands squeaks ahead in this respect, as there are attractions and rides that appeal to both young children and teens, boys and girls, ride wimps like me and extreme coaster junkies. Magic Kingdom, on the other hand, has many more rides for toddlers, as well as ones that are senior-friendly, which brings us to...
Accessibility: Disney is far more accessible to people with 'unusual body dimensions', and also people who are mobility impaired. I don't know how their accommodations for people with special needs compare, but I'll give this one to Disney anyway.
Parties and Special Events: Disney, all the way.
Size: Disney has more rides, more shows, more of almost everything.
I've probably forgotten something, but depending on the value you assign to different aspects of your vacation experience, I could easily see either park coming out ahead.