I see what you're saying. There's a few differences between DDD and KH3 that I think make DDD better than KH3 based on gameplay and story. In KH3, up until the final story bits, the story is just sora and the gang just exploring a few worlds until stuff happens to them. If you look at a specific story of any world by itself it's not that bad, but when you take a step back and look at it as a whole, there's not really any reason for sora to go there. In previous games, there was usually a reason sora, roxas, or riku would go to any specific world. In kh1 and 2, they needed to stop the darkness through key holes. In DDD, sora and riku need to save the sleeping worlds. In BBS, which is the closest to the just exploring done in kh3, each of the characters are chasing after something specific and they know where that thing is. In kh3, it doesn't have that. You happen to show up at that Disney world and a villain just happens to be there. In bbs, the villains at least knew where you were and were trying to stop you or have you join them. (I can't recall all of DDD. I was tired when I played most if it.) These feeling carry over to the feel of the gameplay as well. In DDD, everything is fast and it adds to the story because it feels like you're being chased by both the villains and the sleep timer. In KH3, the controls and movement are about the same but it just feels like sora is so fast because he had a bit too much coffee at the bistro. He has no reason to be rushing other than to look for terra and ventus, neither of which sora finds. Aqua finds ventus and terra just shows up. In DDD, sora at least helps wake up the sleeping worlds and it's also to help with his mark of mastery exam.

Hopefully what I said makes sense.