Unless you have an audio processor that will decode DD True HD, or a player that will decode it, you are better off using the PCM or DD + audio tracks. If your source or audio processor will not decode the DDTHD track, all you get is a down sampled, compressed track with less information than DD+ or PCM.

Also, I just read an article that Murph posted talking about Dialanorm that is encoded with DDTHD. It lowers overall audio playback on all disks so that dialog playback is consistent between different movies.