Wow, that's a tough thing to answer! I've done both (land and sea) in one 8-day trip. It was the best combination.
Cruise pros: No crowds, almost no lines, no massive planning/scheduling (ADR, FPP, which park, TP), easy access to air conditioning/beds/quiet, no buses/driving, much more relaxing, easy to find food at all hours, no concern about cost of food choices (not counting the up-charge dining venues), lots of child-free time while they spend most of the days (and some evenings) in their activity centers, very short commutes (walks) from on-board activities to room/pool instead of long time from park to resort and back, kids can wander/roam the ship on their own at younger age than you'd let them do in a theme park.
WDW pros: Rides, attractions, parades, more meet and greet characters, more than one show per day, sense of being "not in Kansas anymore," more dining choices, not having to pay (and pay big bucks) for each shore excursion (daytime activities not on the ship) vs all rides/attractions included in theme park admission, being "in the bubble" (going ashore any place except Disney's private beach is back into the real world, sometimes not so great), bigger fireworks shows.
I'm sure there are more reasons. Also cost. Which one costs more depends on your choices.