At Disney World, there is more advantage to staying on property than at Disneyland, in part, because if you stay on property at Disney World, they take you back and forth from the airport as an included benefit of your resort stay (they even pick up your luggage and deliver it to your resort so you don't have to wait at baggage claim). It's in their best interest to do this because they want to lock you on their property.

Disney World actually has 3 meal plans (quick service, regular, and deluxe). In actuality, none of them really save you much money versus paying out of pocket but there's something nice about having everything paid for up-front (makes the vacation feel more all-inclusive). The deluxe plan is super expensive, but it's awesome (the best restaurants in Disney World are the sit-down restaurants in the resorts).

Disney World, IMO, isn't really any better than Disneyland for the parks and attractions (there are about equally as many rides in Disney World's four parks as in Disneyland's two parks). In fact, one could argue that for just the parks themselves, Disneyland is better because everything is much closer together. What sets Disney World apart over Disneyland, IMO, is that it's more immersive, has way more resorts and restaurants, and also has two pretty awesome water parks.

FWIW, I agree with others suggestions to take 2 days for Magic Kingdom and 1 day for each of the other 3 parks.