I am still confused which I should be choosing if there are different mixes on a disk
My AVR defaults to DTS-HD Master if it's an option, but I watched something the other night in PCM Lossless rather than the other, dolby digital 5.1 mix on the disk, and I think it sounded better than the DD option when switching between them.
Which is 'better'??
The answer is: it depends.
If all tracks came from the same master, then the only differences will be due to the encoding itself, in which case, PCM should be identical to DTS-HD or Dolby TrueHD, and these should be superior (if only subtly so) than the Dolby Digital encode. I have heard it stated that DD mixes have infrasonic bass rolled off, but I've never seen this claim verified. Note that when comparing the mixes subjectively, don't be fooled by different levels arising from use of Dialnorm in one of the tracks, usually the DD one. If used correctly, Dialnorm will lower the playback level of the track so that it its plays at a level consistent with other properly Dialnormed tracks. In order to make the comparison fair, the non-Dialnormed track should be level matched against the one that is DIalnormed.
There may however be Blu-rays in which different tracks come from different masters. One obvious case of this would be a 5.1 DD mix accompanying a 7.1 mix in a lossless format. There may be other cases, such as a theatrical master in DD format being bundled along with a lossless near-field master. Unfortunately, when the tracks come from different masters, it's becomes harder to judge which track is better. For example, if the near-field mix was done particularly badly, then it is at least possible that the DD track might sound superior to the lossless one. How often this occurs in practice is unclear. It may be fairly rare, so that in most instances, it's still almost always better to choose the lossless track if it's available.