Interesting but very common subject throughout the forums I'm afraid.
Only few songs have some similarities, mostly because those 'romantic' synth atmospheres generated by both Barbieri and Rhodes but that would be it. Their visual style are much more similar (during '81/'82) and I think people are more into comparing them visually than musically.
Actually, the best thing about Japan was that they split pretty fast, clearing the way for some extraordinary and wonderful solo pieces by Sylvian, Jensen, Karn and Barbieri. If they had stayed together, the most of these would never have happened.
DD have been more or less together for more than 30 years now,and for that time they gave almost nothing valuable comparing with Japan and its members.
Now just imagine Japan reunited, releasing their new crappy album, something like 'The Red Carpet Massacre' is!
Don't get me wrong, when I was a teen at early '80s I was a huge DD fan, only listened to Japan briefly, but now, after 25 years and more, I still listen to Japan stuff very often...