K.S.
Hi, first I would say there is a great book buy Birnbaum about Disney trips (for sure Disneyworld, I hope Disneyland). I don't usually resesarch that much, but I was SO glad I got this book- great tips.
We've been to both, and like them for different reasons. I would agree with whoever said Disneyland is the best place to start. It doesn't seem so overwhelming, and you can also get the pass to go into California Adventure.
Disneyworld is the same themes, but is bigger. Plus I think things like EPCOT are more interesting and memorable for older kids, so if you plan to go to both parks, I would do this one later.
You'll have a great time no matter what you decide, they're fantastic places. But really do some planning before you go- map out your days, otherwise you'll be exhausted and waste a lot of time spinning your wheels.