I grew up in AZ and then CA, so I have been to DL many times. I then moved to TN and have now been to WDW several times. They both have some great things about them. Here are my opinions:
WDW:
pros -
Great variety of rides, shows, shopping and other attractions
You can spend longer than a week and never get bored
Wonderful restaurants
cons-
It's tiring riding a shuttle or driving to the parks
It's also tiring walking so much
(those are the only two cons I can think of)
DL:
pros -
Everything is within walking distance
They have many rides that WDW doesn't have and several unique shows
DTD is right there in the middle of everything, you don't have to take shuttles or drive anywhere
If you want to go back to the hotel for a break, it only takes 10 minutes
Also, you can't beat CA weather. I have been all over the US and the weather in CA is some of the best in the country
cons -
They don't have four parks, there are only two, so you will be able to see and do everything in a shorter period of time. No World Showcase, no Spaceship Earth, etc. Although they have much more in DL than in the Magic Kingdom.
All in all, I enjoy going to both places. I look at it this way. For a shorter vacation without all the running around, DL can't be beat. Everything is right there and it's never a hassle to get there. If you want a longer vacation, there are many fun places to visit while in CA.
WDW is a great all-inclusive type place to go for a week or more. They've got such a wonderful variety of things to do that you never need to plan going anywhere else. (We also visit Universal/IOA for a couple days on our trips though).
We are going to DL for 6 nights this Oct. Then to WDW in March. It's kind of like having the best of both worlds!
