Probably DD, as Nightwing didn't show any combat feats at all in that movie. But I think it'd be a sweet fight. Both of them were making crazy jumps from building to building and tanking those "How the bloody hell did he not shatter his legs" falls.
Not sure about Red Hood though, he's pretty much Batman lite with all his crazy toys and stuff that could offset DD in some way.
Under The Red Hood was a ****ing sweet flick too. These animated Batman movies are getting better and better.