San Diego things to do:
Sea World is more like an amusement park. There are more rides, and it's good for someone who doesn't have a great attention span (I'm not saying that's you, ok?!!!) If you have only 3-4 hours, you'll feel that you accomplished a lot.
Education-wise, Sea World has an obvious edge in the aquatic, marine life department. So, you have to factor that in if you like sea animals.
The Zoo has a lot more to offer, if you have more time. The Park is huge, and there's just so much more to take in -- not just animals, but plant life, two immense bird "cages", and yes, there are shows. Parking is free, unlike Sea World.
Outside the Zoo, you've got Balboa Park, with museums, the planetarium, space theater, and nice restaurants.
If you are coming from the north (or staying in the northern part of San Diego), there's the San Diego Wild Animal Park -- and that's even bigger, with animals living in their own natural habitat.
Which one is better? Personally, I'd go with the Zoo. There are Sea Worlds in other states, along with other marine parks. But there is only ONE San Diego Zoo. Very few zoos in America compare to San Diego.
A good site for things to do in San Diego and San Diego hotels is:
http://www.san-diego-vacation-attractions-tours.us/san-diego-lodging-hotels.htm
http://www.san-diego-vacation-attractions-tours.us
http://www.san-diego-vacation-attractions-tours.us/san-diego-events-activities.htm
http://www.san-diego-vacation-attractions-tours.us/san-diego-legal-websites.htm
http://www.san-diego-vacation-attractions-tours.us/san-diego-internet-websites.htm
http://www.web-site-award-winning.com/