I absolutely love both parks, but have to vote for IOA by a very bare smidgeon. I am absolutely NOT a thrill ride person - have never ridden Dueling Dragons and have only attempted Hulk once, but I love the attention to detail there, right down to the trash cans. I love how the music changes when you move from land to land. And there is no better themeing than Seussland
My littlest one has preferred to go to IOA since he was 4. Loves all the rides and can play for hours in the Jurassic Park play area. He was so thrilled when at 8 he could finally get on Dueling Dragons. So, that is what swayed me to vote for IOA, my 9 year old's opinion

Although, the mere memory of having to ride Unicorn 12 times in a row might just make me change my mind and vote for MK.
The shows at IOA are great and I was surprised at how many rides I actually enjoyed and how much there was to do for my youngest kids. Spiderman and Popeye are my favorites.
I have to admit, I was disappointed when MK started removing rides like 20,000 Leagues Under The Sea and then didn't replace them.
But I have to agree with the other posters. You really can't compare the two. They are different parks and offer different experiences, both of which are fantastic. I am just glad that I have 6 fantastically put together parks (sometimes 7 when we include Sea World) to enjoy when we head on down to Orlando. I love them all.
And having both in Orlando just makes everything better, because they keep each other on their toes. Better for us, the consumer.