Having played both games, I have to say DD comes out on top for me. I like OMD a lot, and it's a great singleplayer game. The humour is top-notch. Eventually though, it's going to be more or less the same thing again.
DD is less charismatic, and the interface is an example of "too much flash over substance", but it's got a lot of lasting appeal. Even when playing solo, I enjoy it a lot, as there's a lot of strategy involved, and most towers/abilities have some use one way or another, so it offers diverse amounts of strategy. The hack n slash is also very solid. It's just that, well, like I said before, it's not a very charismatic game. The enemies feel wooden and cardboard (unlike the orcs in OMD with their funny one-liners like "Can I be the good guy now?"); the main characters, however, are very likable. I also miss having a sprint function :( Some of the levels in DD are also downright boring. I mean, they're just not fun to play; yet, there are some that you will find yourself playing over and over again, and not just to grind for better loot - they're really a lot of fun to try out new strategies.
All things considered though, DD comes out on top for content. Still, if you can get both games, just do it. They're different enough so you still get your money's worth. Just bear in mind that OMD's lasting appeal only goes as far as its campaign's length.
