Comparing the Formats
If you have a choice, listen to the Dolby Digital or DTS soundtrack on a DVD, and skip the Dolby Pro Logic soundtrack. That's because either of the discrete formats have much better channel separation than you find with matrix surround. The surround channel in Dolby Surround and Dolby Pro Logic soundtracks is often indistinct and always nondirectional; it sounds mushy, and has a fairly narrow dynamic range.
In comparison, all the discrete surround formats have very clear surround channels, with distinct channel separation. Listen to The Matrix in Dolby Digital and you'll clearly hear bullets shoot from right to left behind you, something that just isn't possible with Dolby Pro Logic. An even better test is the subtle surround effects on the Days of Heaven DVD; in Dolby Digital, you can hear the soft rustling of the wheat fields and the constant droning of the crickets enveloping your listening position. The discrete formats are also better at reproducing the reverberation of a concert hall, as witnessed on the James Taylor: Live at the Beacon Theater DVD. It really feels like you're in the middle of the audience.
If you have the choice, I recommend listening to the DTS soundtrack over the Dolby Digital version. The DTS version of Saving Private Ryan features much more realistic rear channel effects than the Dolby Digital DVD; the whizzing bullets are more clearly positioned in the sound field, and the explosions have a greater depth. The aforementioned Twister is also more terrifying in DTS version than in Dolby Digital.
For the ultimate surround experience, however, you have to add another speaker and use a DTS ES Discrete decoder. I find Dolby Digital EX a tad gimmicky, the additional rear speakers more useful for ambient noise than anything else. The discrete rear channel in DTS ES Discrete, however, delivers the goodson those rare occasions where it's available.
So, in order of sound quality, you start with Dolby Surround and Dolby Pro Logic, make a big leap up to Dolby Digital 5.1, get slightly better results with DTS, and then reach the top level with DTS ES Discrete.