First, hello and welcome <img src="/ubbthreads/images/graemlins/wave.gif" alt="" />
Ok, DD and Dungeon Sige are as different as Sumo and Kickboxing, both are RPG games but one is tactical/party game, the other is action-rpg (which means you're alone in it). In DD you can be aided by summoned monsters but that's all. DD is has more roleplaying, items and the story is fine. Also DD is in 2D. It's pretty difficult to sum all the advantages of DD in one post, maybe someone like DAD will do it, I don't know.
EDIT: How can one possibly be in Germany/USA?