The DDE must be purchased by a FLResident or a passholder. So unless you have an AP you must purchase one (an AP that is), and be sure to do the math to see if it is worthwhile.
The dining plan is a better deal in that you have more food, but on the DDE you have a lot more flexibility.
I did the dining plan for the first time a couple of weeks ago (Spring Break) and have to say that I was not thrilled. I found there to be too much food for us, and while we ate pretty well we spent a lot of time working our days around the ADR. I even did an 8:40pm at Boma because we were not hungry at 5pm after the big CS lunch at noon.
So for me and my kids, its the DDE card and signature dining. But we also go to Disney six times a year. If I were the typical Disney guest with a six night stay once every year or two I would probably do the dining plan. You really do save a lot of money with the DDP. But you eat a lot too.