This is always the subject of friendly debate in this forum. Some people swear by hoppers, and they like to have complete flexibility and variety on each day of their trip. I'm in that camp... one park in the morning where I book my FastPass reservations, leave in the afternoon heat for a second activity (resort, Disney Springs, water park, etc), then a different park in the evening -- especially if there are extra magic hours or a later close than my morning park. Plus, I love being able to add spontaneity to a trip, repeat your favorite attractions, or to see a different nighttime spectacular.
Others would rather save the money, and don't see much need for changing parks. At Disney World, the parks are so spread out that you'll easily spend over an hour walking to the exit of one park, waiting for transportation or finding your car, standing in security, and finally entering another park. Many believe you can find more than enough to do for a full day in any the four big theme parks. Also, it's often said that you can always add hoppers onto your ticket later instead of bearing that expense upfront.
Personally, I'll always go with hoppers on a multi-day Disney trip, unless budget dictates otherwise.