Having been to both MANY times (when we lived in Seattle BEFORE we moved to Florida), WDW is by far our favorite.
Why?
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Stuff to do. TONS of stuff to do. Resorts, restaurants, 4 parks, water parks, more more more.
Disneyland just doesn't compare, sorry.
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Immersion. You feel like you are in a magical world day after day. There is no outside city that you can see for miles and miles. In Disneyland just go up into Mickey's Fun Wheel and you can see that city everywhere. Not so in WDW.
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Magic. Honestly, Disneyland has more locals visit it, which takes away a lot for me (funning me saying that now being a local in FL). It also has a lot of local minimum wage Cast Members. EPCOT has Cast from the actual countries! I feel like the people here are nicer and better managed, including better queues and wider walking areas.
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Stuff OUTSIDE of WDW. All of the outside theme parks (and stuff to do in general) is better in Florida than their counterparts in California. This includes Universal Studios, Legoland, water parks, beaches, etc. Even things like traffic is better.
Are there advantages to Disneyland? Sure. Weather is one if you are taking a summer vacation. There are others too. But WDW is part of the reason we moved from Seattle to Florida instead of Seattle to California.
By the way, these are just my opinions and you can disagree. They are not set in stone.
Ethan