Duals can store more information than Singles. Now that we have that out of the way....
Dual and Single layer dvds have no DIRECT impact on the video/audio quality of the movie. However, the less compression the movie suffers, the better the quality. So, more space = less compression = better video quality.
DTS is the preferred choice for most people but the DTS format takes up a lot of space, that's another reason why most (all?) DTS dvds are dual, for better picture and to accomodate the DTS feature, same goes for the number of special features. More space means a larger accomodation of "extras".