Well, let's put it this way. DTS sounds better simply because there is much less compressian than Dolby Digital. However, unless you're running a $25,000 audio system you may be hard pressed to see any significant differences.

One thing to consider is that typically the "good" receivers come with Dolby Digial and DTS. Perhaps even THX. Really, DTS isn't that expensive of an option. If you have it, you'll always be covered. One thing is if you plan on keeping the receiver for a number of years you may want to consider one that also has 6-channel inputs for future sound formats, say a home version of SDDS.

Despite the hype over Dolby Digital EX, I really wouldn't worry about it.