essentially they are the same. both are digital with 6 channels.
HOWEVER, DTS uses less compression, and thus has better sound quality. But DD sounds just fine, nothing really wrong with it. few people will bother with the difference.
if you want high end, get something that can do both, receiver, decoder, DVD player.
realize that dd is the dvd audio standard. and dts is a feature on some dvd movies, and is a feature some look for.
actually, if anyone was in the market, i'd recommend a dts and dd system.