We are doing both, but let me explain. We're arriving in WDW on Sunday (1/22) and activating our annual passes. The DDP isn't available to us yet, so we're going to be getting the DDE to use over the next year. However, we do plan to use the DDP on our September visit. But if we choose to eat off of the plan we can still use our DDE. When we return again next January we'll probably skip on the DDP again because my parents will be with us and they generally buy a lot of our meals and won't want the dining plan, but we can give them the discount. Also, if we make any spur of the moment trips over the year (dh may have to work in Orlando occasionally....) we'll use the DDE card as we won't be eligible for the DDP. When I can get it I like the dining plan because its not all sit down restaurants. With 3 kids sometimes that's hard!
Granted, our savings on the dining plan won't be as great as it would be if we didn't have DDE, but we'll still see some savings from my calculations....and the DDE will more than pay for itself in the few meals we have planned for this trip.