Good luck, Monsier Song. Either choice is a compromise, a work-around solution that responds to limited space and money. Drumming is a secondary instrument for me (I've been a session guitarist for many years), but the dd55 works better for me than a drum machine, primarily because I like to "play" my drum parts if I'm working at home. The moment I have a large studio space and money to spare, both of which will come, I'll be picking up a real set of drums. No question there. Until then, I'm satisfied with what I have and can recommend it, but only for special cases like mine.
By the way, speaking of discount stores, I hear that the dd55 is selling at Cosco for $200, stand included. Two of my recording colleagues have already picked one up there although they probably wore a disguise while doing so. ("For me? No way! It's just a stupid Christmas present for my kid...")
Let us know how it all goes for you, eh?
J.