which audio to use PCM or Dolby HD

[Anthony P]

Does the dobly HD sometimes have a higher bit rate

in this case bitrate does not mean anything. Digital sound has two metrics. One is called word length and the other sampling frequency

The word length can be thought of as precision the values usually seen are 16 and 24 with 20 in third place. this explains how much bit precision there is in every sampling of the sound. (the bigger the better)

The Sampeling frequency. (a number like 48, 96 or 192). Sound like motion is continuous. The same way film cannot really show the motion and takes still pics equally interval, sound samples are taken at equal intervals. The DAC on your equipment at home will then take these pics and try to recreate the true motion of the sound wave.(the bigger the better)

For example most Disney titles have that info on them like (48kHz/24 bit)

Like NutsAboutPS3 pointed out on some titles the DTHD came from a better digital master.

The bitrate does not matter and the answer to the question as asked is no, if the PCM and DTHD used the same digital master DTHD <= PCM. But if the DTHD came from a better master then in theory it can happen, but it is unlikely.

If you start from an analog source you then get a digital master depending on the values inputted for word and frequency. This digital master is PCM and has no compression so is of a constant bitrate (simple calc since we have the word(bits)*sampling frequency (samples/second)* channels). Then that digital sound is compressed into DTHD.

For all intents and purposes, a PCM track is what a sound mixing studio would be using to master a movie soundtrack.

not necessarily, most are done in analog. The thing is that since DTHD is compressed it takes less BW and less space on the disk so a studio could go with a better digital Master then the PCM.
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personally, because of the reasons mentioned by others my choices are

1) highest bit/frequency
2) if the same pick PCM over DTHD