they both have advantages and disadvantages :-
Disneyland, no hassle to drive (as i live in NT) and with gold membership annual pass, free car parking, better for younger kids and pregnant women (no hills), okish rides, better than before, argueably marginally better public transport than ocean park, BUT food is crap and expensive
Ocean Park, aquarium, better rides for older kids and adults, real animals, better food, good transportation esp if you live in HK island, BUT the location makes it a pain to drive on weekdays if you live in NT or Kowloon as the traffic is quite bad,