Of course Dts sounds better on DVD because it uses a higher bit rate and less compression than Dolby.
On Blu ray there supposedly is not much of a difference since both are HD audio lossless.
Not the one to really say as i don't have blu ray, but that is what i take out of my online reading.
When playing back ac3 tracks on DVD you have to make sure the dynamic range compression is turned off. Makes a huge difference even with titles like star wars gout.
DTS was better on DVD when they did full bitrate then they did half. Many people used to import titles from overseas because the DVD's from Japan would not be half bitrate. Before the advent of Blu Ray superbit was the way to go for DTS DVD.