DTS uses a different mix than DD, thats why it sounds different. The compression on DD seems to be quite capable, especially when you consider that most movie soundtracks dont have as much going on as music does, it doesnt need as much compression. Most DD 5.1 mixes are comparable to a 192k mp3, and as I said, when you consider that the frequency range of each channel doesnt compare to music, that is more than plenty. DTS is probably overkill.
The DTS mixes that I have heard, in comparison to DD, only have more bass and louder surround channels, in addition to having more gain over the entire track. I would love to compare a DTS and DD mix of the same source, but I have yet to find one. DTS to me is just a waste of space and the equivalent of harshly compressed CD audio, only worse because of the bandwidth constraints of DVD force the video to look worse.