DTS encoding has always used a higher bitrate than DD. Whether or not that improves the sound is arguable. and people have argued about it for a long time, since DD and DTS use quite different encoding methods.
My understanding is that DTS sound levels usually are recorded at a somewhat higher volume than DD sound levels when both tracks are available on the same disc. When there are slight volume differences, people tend to claim that the louder audio sounds better: very careful matching of sound levels is needed in order to make an unbiased choice.
Both types of audio encoding have a "core" lossy audio stream plus additional audio streams to improve the quality of the audio. A DD+ audio data can be fed to a standard Dolby Digital decoder, which will ignore the "plus" stream and decode just the core audio stream. A DTS decoder cannot decode any of the Dolby audio formats, and, similarly, a DD decoder can't process DTS. (With some of the early Dolby receivers, you had to be very careful not to send them DTS audio data, since doing so would generate a very loud, scratchy noise, potentially damaging the speakers.)
