Most importantly, DDG comes with a host of bangs, whereas firefox And I assume chrome as well) require some configuration. Secondarily, bangs work regardless of their position in your query. At least in Firefox, they must be the first word. Finally, I can use DDG with bangs from anyone's device, that I might not want to do any setup on.

I do primarily use bangs from Firefox's address/search uni-bar, so if they added bang support from there, I would start using that feature without changing my habits at all.