I've done DID at Epcot and it was great. There is not extensive time with the dolphins, but it is fun. Discovery Cove offers more actual swim time, if that's what you want. DID is designed to be more educational.
One tip: book EARLY in the week. Mondays are best because the dolphins have had no outside contact except CMs all weekend. Friday would be the "worst" because they've seen people all week long. However, I've never heard of them refusing to interact. Earlier in the week, they may be more anxious to meet and greet visitors.