AFAIK, going for level 15+ in DD is worth it, since it gets you additional attacks (wings, along with the bite attack in the first 10 levels ..those attacks work even if you use weapons - i think-) AND the size increase a bit the damge, but the most important, it allows you to use some 2 handed weapons (great sword) as one handed ..meaning you can dual 2 great swords at the end
or use one GS with a shield.