difference between Dolby Digital 5.1 and DTS 5.1?

Both are lossy compression systems for 5.1-channel digital data streams. DTS compresses less and retains more of the original info, so generally sounds better.
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Actually the compression ratio for DTS and DD on DVD are the same. IIRC it something like 12:1 DTS got the reputation for using less compression back in the days of laser disc. On a laser disc DTS is something like 3:1 vs DD's 12:1.

Anybody who claims they hear something different is right, but it's not due to compression. It's different mastering. The soundtracks are done in different studio's by different technitions. The DD soundtrack is the same master track as the theater, the DTS is a remaster.

As an example, look and listen to Gladiator. You can argue the the dts and dd sond differnent, but you can't argue with the fact that the DD soundtrack won an Academy Award and the DTS did not.

Don't get me wrong, I like having both on my disc's.

The DTS track is always a larger file, how could they be the same? You will only need to strip both soundtracks to see this.

Dave