difference between Dolby Digital 5.1 and DTS 5.1?

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 key phrase in Markphd statement is "In theory it is superior." This is true, in theory it is supperior. However it is all relative to the source. The maximum bit rate for DVD's with mutiple soundtracks is 448 kps acording to Dolby and the DVD liscence agreement. However if the disc is a single soundtrack it can the use up to 754kps. This is the case on some superbit disc and special dts only editions (jaws,saving private ryan, etc.)

It is not a matter of what they are capable of doing, it is what they are limited to.

As far as the DTS soundtrack being a larger file than the DD file, it is because they are differnt file types. IE different languages and differnet decoders. They file size is not the issue in compression, it's the bit rate.