That's like asking which M&Ms do you like the most.
I enjoy both the plain and the peanut M&Ms, and even the almond M&Ms.
As to which when, it depends on the time of year, crowds, weather. Disneyland tends to have the most moderate weather, with Disneyworld being down right unbearable in the summer. Disneyland is also more concentrated, with Disneyworld being more spread out. But Disneyworld is where the company is spending the most money on new attractions.
Disneyland has the home state advantage, so we can visit far more often, but if we lived in Omaha, I'd probably visit Disneyworld far more, though I'd miss POTC of Disneyland...