Heh, this debate has been done to death, but yeah, really DTS is just encoded louder, usually around +10 db. Dolby has a more effecient encoder, and if used properly, can achieve identical/better results than DTS. Unfortunately, a lot of 5.1 DD soundtracks on DVDs are mixed so that they still sound ok when someone without a 5.1 setup plays back the track on their TVs, and the DVD player has to downmix the DD track for 2 speakers. It's a shame really.
The only time that you can claim the DTS track actually sounds signifigantly better, is when there are different masters/mixes used for the tracks, all other things being equal. When the DTS company used to handle the mastering process themselves, they "cooked" the DTS tracks so they sounded very different from their DD counterparts. See Saving Private Ryan as a prime example. Also, the Gladiator and Jurassic Park (corrected DTS version) DVDs come to mind.