We had a 4 night stay a couple of weeks ago. We kept track of our dining expenses for the two of us and we saved about $25 using the DDE card over what the DP would have cost us.
The comparison for your group needs to be based on how you would normally dine. If you usually have a counter serve and a full service meal each day- the DP will likely save $$. In our case, we rarely have an appetizer or dessert and do not have a full service meal each day. On this trip we had breakfast in our villa each day (cereal, juice, milk and grapefruit purchased at the Conch Flats store - ~$15 total), ate at the Brown Derby, Olivia's and Narcoossee's (using DDE) and also had three counter-serve meals (none accepted the DDE). Our totals for those meals (including breakfast) was about $280 including gratuity and not including "adult" beverages. Cost for the four days with the Dining Plan would have been about $304. When you figure in the discount on the wines we enjoyed with our dinners, the savings would have been even more.
Just do the math based on your usual dining habits as this is not a one-size-fits-all decision. In our case, the DDE will serve us best, but for those with different dining habits, the Dining Plan will be the better choice.