DDE if you are light eaters(no apps or deserts needed), or plan on doing mostly TS. To me the 20% is just saving the tip, but not having to get all the extra food is helpful too.
DP if you can make use of the CS even if you are going to use them for breakfast and a few lunches. The TS's can always be split. 2 adults do not have to order 2 complete meals-exceptions are the buffets ofcourse where everyone pays. Other thing even if you plan on doing more TS and can make uses of a few CS's...is that many lunch TS can qualify for a AP 10% discount...besure to check this out-what restaurants participate.
To me the DP is fantastic...I split our Apps with our children who are 2 weeks away from 9yo on our last trip. And counter cs lunches I would also split with 1 child...leaving a credit for breakfast(at Broadway bakery or the BC marketplace or MS bakery). But the true benefit comes in that the tip is included...this is a huge savings IMO. Snack credits I used to grab a bag of bagles and 2L bottles of soda for the room. We were never hungry, but never felt like I was rolling myself around either..thanks to the splitting.
Good luck in what ever you decide.