This was the first trip to WDW that I had the TIW card and did not use the DDP.
I had 5 other adults in my travelling party and 4 of the 5 enjoyed a cocktail before dinner. That alone saved a fortune with the TIW card. Coupled that with the restaurants we chose (and the few QS that honor TIW) and I found that I saved a lot more with the TIW card than I did from the DDP.
For a few years we swore by the DDP - it was convenient was the best aspect. We had prepaid meals and didn't have to worry about carrying cash or credit cards for meals. But along with that I kept every receipt and logged how much we 'saved' over going out of pocket. What I found was that the 'up to 40%' savings Disney touted was really an illusion.
Here's part of the reason why - the DDP includes dessert for both the QS and TS. It wasn't something we normally would eat....most of my travel partners preferred appetizers rather than dessert. But since it was included we took it (and often took it back to the room). So we still ordered the appy, which increased our costs, and took the dessert even though we normally wouldn't. This extra dessert cost when factored into the 'savings' turned into costs that usually would not have been incurred. Even with the desserts, I found that my savings were never more than 5 - 10%.
So.... it really comes down to how you and your family prefers to eat. Do you enjoy a drink or two? Those savings add up quickly with TIW. Do you enjoy an appetizer? Again TIW savings.... If neither of these are part of your usual dining experience, and you like dessert, the DDP might fit your needs best.....