I personally prefer DTS.
I've not listened to very many DTS movies, but those that I have heard sound noticely better than DD 5.1. The DTS audio has a more natural feel to it, whereas the DD seems to have more separation between the channels. DTS blends the channels together nicely. I just watched Master & Commander last night in DTS after reading about it in this forum. Great sounding movie.
I have only had my DTS receiver for a couple months now, but now the first thing I look for on DVDs is that lovely DTS logo.
I do agree with Leprkon though, the effectiveness of either is in the hands of the person mixing the audio.