The game isn't shorter, they're both about the same. If you try to finish each individual quest and explore the map a bit, they're both good for about 100 hours each.
The reason BD is on 2CD's is because the characters aren't prerendered anymore. Though the models are slightly less detailed, the animation is alot smoother, and 3D models don't take up as much space as thousands of little pictures.
Also, since BD renders characters on the spot, you can dress up your character better.
Like in DD when you equipped a pot on your head, it would look like any other metal helmet, while in BD it actually looks like a pot <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
Tovi 