The reason DTS sounds better is because its a 1.5 mbit/stream bitrate to each of the 6 channels. I'm not sure what DD is but I think its around 256k. Either way you can't hear the diff unless you have a good home theater system.
Another thing is, many movies don't really do 6 full channels of Dolby Digital even if it says so on the box. Especially the older ones. A lot of the time the studio translates the Prologic soundtrack to DD, matrixes the front channels to the rear, and leaves out the sub channel. The Top Gun DVD is a good example of this.
With DTS, the movie itself has 6 full distinct channels encoded into the soundtrack so you always get exactly what was originally done, not a cheap remix.