The parks at Walt Disney World are really very, very, very nice parks. But after seeing their "best" rides represented in a "best of" format at Disneyland, it's odd to see them so spread out. You can do Disney World in 2 or 3 days (or less!) if rides are what you're after, and I don't doubt you'd leave with fantastic memories as a result. But if you wanted a vacation, and a chance to take in the details and see the "best kept secrets" of the parks, you'd want more. Disney officials would have you believe that half of the fun of Disney world is in Downtown Disney, water parks, shopping, dining, etc. I bet it's true, if you had the time and energy to do it all.
Universal packs a lot into its two parks and does so with more charm than people may give it credit for (Seuss Landing is really just incomparable, in my opinion). If you haven't been there in a decade, you may as well not have been there. More than any other park I've experienced, Universal has no qualms about replacing classic attractions with whatever the newest, hottest title is (which has been good and bad in the past). As was said, they've got Spider-Man, Jurassic Park, Harry Potter and the Forbidden Journey, Dueling Dragons Challenge, Hulk, Cat in the Hat, Revenge of the Mummy, Men In Black: Alien Attack, Disaster, and the oddly fun E.T. Islands of Adventure is on-par with and even excels beyond any of Disney's domestic parks in terms of theme and detail.
There's a lot to think about! I'm happy to help if you have questions.