Well, before you go, do some research. Look at what attractions the parks have to offer. A lot of them are shared between each other, especially with the Magic Kingdom, but a lot of them are not. Look at what's available to do outside of the Disney park in the surrounding areas. Is this going to be your whole trip or just one part of it? If its only going to be for a couple of days, then Disneyland park hopper might be a better option since you'll be able to see much of it in one day, then go back and do the things you either missed or wanted to do again. Disneyland also has some historical value as the first-ever Disney Park, as well.
I spent at least three days at Disney World back in 1997, and that was before the Animal Kingdom park was ever open. Nivia23 forgot to mention that Disney World also has a couple of waterparks you could visit as well. If you are planning to be spending much more time at a resort, then Disney World is almost certainly the best option.
Either way, you can't go wrong. Whichever one you choose, you will certainly have a great time.